こんばんは
libretto L5 についているFaxモデムに使われているチップが分からないと何とも答えようがありませんが、普通の常識ではWinmodemはリナックスでは動作しません、としか答えられないと思います。
RS232C接続のモデムならなんとかなると思いますが、内蔵の場合はふつうはWinmodemといって、ウインドウズ本体がドライバーを持っていて、Linux用のドライバはありません。ただ、チップによっては開発されたものもあるようですので、ネットで検索してみてください。
参考；faxmodem Linuxで検索すると情報が得られます。（例：http://www.yk.rim.or.jp/~kawagisi/fax.html）

モデムのチップは
sudo cat /proc/pci
sudo scanpci -v
sudo /sbin/lspci -v
等で確認できるそうです。
私が確かめた情報ではないのですが、参考にしてください。Linuxでのドライバーが見つかることを祈ります。

もう少し調べてみましたが、
Linux用のドライバが添付されているファックスモデムはラトックの"REX-PCI56CX"(http://www.rexpccard.co.jp/products/sub … i56cx.html)しか見つかりませんでした。

funatogawa様、petitbootang様
情報ありがとう御座いました、取り急ぎscanModemを実行してみた結果が次になります。
ALI5451がモデムに使われてるようでした。


$ cat ModemData.txt

Only plain text email is forwarded by the  [email protected] List Server,
as HTML can contain viruses. Use as the email Subject Line:
           YourName, YourCountry  kernel 2.6.31-16-generic
With this Subject Line cogent experts will be alerted, and useful case names left in the Archive.
YourCountry will enable Country specific guidance. Linux experts in YourCountry
can be found through: http://www.linux.org/groups/index.html.
They will know your Country's modem code, which may be essential for dialup service.
Responses from [email protected] are sometimes blocked by an Internet Provider mail filters.
So in a day, also check the Archived responses at http://www.linmodems.org
--------------------------  System information ----------------------------
CPU=i586,
Linux version 2.6.31-16-generic (buildd@rothera) (gcc version 4.4.1 (Ubuntu 4.4.1-4ubuntu8) ) #53-Ubuntu SMP Tue Dec 8 04:01:29 UTC 2009
scanModem update of:  2010_01_22

The dialer utility package WVDIAL does not appear to be installed on your System.
For Ubuntu users, there are at the bottom of http://linmodems.technion.ac.il/packages/
packages with the files necessary to install wvdial, with names like:
     wvdial_jaunty_amd64.zip   for x86_64, 64 bit bus systems.
     wvdial_jaunty_i386.zip    for 32 bit systems.
     wvdial_karmic_i386.zip    for 32 bit systems.
These are about 1 MB in size.  After downloaded and copied into your Linux partition:
$ unzip wv*.zip
Within the new folder:
$ sudo dpkg -i *.deb
will  complete the wvdial installation
Please read Modem/DOCs/wvdial.txt for usage information.

Presently install your Linux Distributions dkms package. It provides for automated driver updates,
following upgrade of your kernel.  For details see http://linux.dell.com/projects.shtml#dkms

There are no blacklisted modem drivers in /etc/modprobe*  files

Potentially useful modem drivers now loaded are:
     snd_ali5451

If a USB modem or cellphone is attached and was not detected, please
provide available information in your request to [email protected]

For candidate card in slot 00:06.0, firmware information and bootup diagnostics are:
PCI slot   PCI ID              SubsystemID     Name
---------- ---------   ---------       --------------
00:06.0    10b9:5451   1179:0001       Multimedia audio controller: ALi Corporation M5451 PCI AC-Link Controller Audio Device

Modem interrupt assignment and sharing:
10:          0    XT-PIC-XT        ALI 5451
--- Bootup diagnostics for card in PCI slot 00:06.0 ----
[    0.775694] pci 0000:00:06.0: reg 10 io port: [0x00-0xff]
[    0.776139] pci 0000:00:06.0: reg 14 32bit mmio: [0x000000-0x000fff]
[    0.776298] pci 0000:00:06.0: supports D1 D2
[    0.776328] pci 0000:00:06.0: PME# supported from D2 D3hot D3cold
[    0.776471] pci 0000:00:06.0: PME# disabled
[    0.971860] pnp 00:08: io resource (0x62-0x62) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972040] pnp 00:08: io resource (0x66-0x66) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972157] pnp 00:08: io resource (0x80-0x80) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972269] pnp 00:08: io resource (0x84-0x86) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972382] pnp 00:08: io resource (0x88-0x88) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972492] pnp 00:08: io resource (0x8c-0x8e) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972605] pnp 00:08: io resource (0x92-0x92) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972709] pnp 00:08: io resource (0xb0-0xb3) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[    0.972812] pnp 00:08: io resource (0xe0-0xef) overlaps 0000:00:06.0 BAR 0 (0x0-0xff), disabling
[   59.715609] ALI 5451 0000:00:06.0: power state changed by ACPI to D0
[   59.732695] ALI 5451 0000:00:06.0: enabling device (0000 -> 0003)
[   59.735995] ALI 5451 0000:00:06.0: PCI INT A -> Link[LNKH] -> GSI 10 (level, low) -> IRQ 10

The PCI slot 00:06.0 of the modem card may be disabled early in
a bootup process,  but then enabled later. If modem drivers load
but the  modem is not responsive, read DOCs/Bootup.txt about possible fixes.
Send dmesg.txt along with ModemData.txt to [email protected]
if help is needed.



===== Advanced Linux Sound Architecture (ALSA) diagnostics =====
The ALSA packages provide audio support and also drivers for some modems.
ALSA diagnostics are written during bootup to /proc/asound/ folders.

The ALSA verion is 1.0.20
The modem cards detected by "aplay -l"  are:
card 0: A5451 [ALI 5451], device 1: ALI 5451 modem [ALI 5451 modem]

The /proc/asound/pcm file reports:
-----------------------
00-00: ALI 5451 : ALI 5451 : playback 32 : capture 1
00-01: ALI 5451 modem : ALI 5451 modem : playback 1 : capture 1

about /proc/asound/cards:
------------------------
0 [A5451          ]: ALI5451 - ALI 5451
                      ALI 5451 at 0x1000, irq 10
Driver snd-ali5451 supports both audio and modem functions of the ALI5451 card.
see http://linmodems.technion.ac.il/bigarch/archive-nineth/msg02665.html about ALSA version issues
If not a Conexant modem, the driver snd-ali5451 with its dependent drivers:

----------
provide audio + modem support with the modem chip residing on the subsystem.
Any particular card can host any one of several soft modem chips.

=== Finished firmware and bootup diagnostics, next deducing cogent software. ===

Predictive  diagnostics for card in bus 00:06.0:
        Modem chipset  detected on
NAME="Multimedia audio controller: ALi Corporation M5451 PCI AC-Link Controller Audio Device "
CLASS=0401
PCIDEV=10b9:5451
SUBSYS=1179:0001
IRQ=10
SOFT=10b9:5451.MC97
CodecArchived=SIL27
IDENT=slmodemd
SLMODEMD_DEVICE=hw:0,1
Driver=snd-ali5451

For candidate modem in:  00:06.0
   0401 Multimedia audio controller: ALi Corporation M5451 PCI AC-Link Controller Audio Device
      Primary device ID:  10b9:5451
    Subsystem PCI_id  1179:0001
    Softmodem codec or chipset from diagnostics:
                               from    Archives: SIL27



Support type needed or chipset: slmodemd

An ALSA (Advanced Linux Sound Architecture) modem driver:  snd-ali5451
provides Low Level support enabling contact with the modem hardware.
For all BUT Conexant chip soft modems (using hsfmodem software)
complementary High Level support is through a Smartlink utility:  slmodemd

Download from http://linmodems.technion.ac.il/packages/smartlink/
the package SLMODEMD.gcc4.4.tar.gz having a compiled slmodemd. Unpack under Linux with:
        $ tar zxf SLMODEMD.gcc4.4.tar.gz
and read instructions therein. But briefly, the modem is setup with command:
        sudo slmodemd -c YOUR_COUNTRY --alsa -shw:0,1
reporting dynamic creation of ports:
        /dev/ttySL0 --> /dev/pts/N   , with N some number
Read DOCs/Smartlink.txt and Modem/DOCs/YourSystem.txt for follow through guidance.

Writing DOCs/Smartlink.txt
============ end Smartlink section =====================

Completed candidate modem analyses.

The base of the UDEV device file system is: /dev/.udev

Versions adequately match for the compiler installed: 4.4.1
             and the compiler used in kernel assembly: 4.4.1



Minimal compiling resources appear complete:
   make utility - /usr/bin/make
   Compiler version 4.4
   linuc_headers base folder /lib/modules/2.6.31-16-generic/build

However some compilations and executable functions may need additional files,
in the FileNames.h (so called kernel "h"eaders) collection installed in  /usr/include/ .
For martian_modem, additional required packages are needed. The also required headers of package libc6 are commonly installed by default.
Compiling hsfmodem drivers does require linux-libc-dev and libc6-dev packages, for kernels 2.6.24 and later versions.
In not included on your install CD, search for them at http://packages.ubuntu.com
or comparable Repository for other Linux distros.
When compiling ALSA drivers, the utility "patch" will also be needed.




If a driver compilation fails, with message including some lack of some FileName.h (stdio.h for example), then
Some additional kernel-header files need installation to /usr/include. The minimal additional packages are libc6-dev
and any of its dependents, under Ubuntu linux-libc-dev

If an alternate ethernet connection is available,
$  apt-get update
$  apt-get -s install linux-kernel-devel
will install needed packages.
For Debian/Ubuntu related distributions, run the following command to display the needed package list:

Otherwise packages have to be found through http://packages.ubuntu.com
Once downloaded and transferred into a Linux partition,
they can be installed alltogether with:
$ sudo dpkg -i *.deb


Checking pppd properties:
        -rwsr-xr-- 1 root dip 277352 Feb 21  2009 /usr/sbin/pppd

In case of an "error 17" "serial loopback" problem, see:
    http://linmodems.technion.ac.il/linmodems/archive-sixth/msg02637.html

To enable dialout without Root permission do:
        $ su - root  (not for Ubuntu)
        sudo chmod a+x /usr/sbin/pppd
or under Ubuntu related Linuxes
        sudo chmod a+x /usr/sbin/pppd

Checking settings of:   /etc/ppp/options
asyncmap 0
noauth
crtscts
lock
hide-password
modem
proxyarp
lcp-echo-interval 30
lcp-echo-failure 4
noipx

In case of a message like:
   Warning: Could not modify /etc/ppp/pap-secrets: Permission denied
see http://linmodems.technion.ac.il/bigarch/archive-sixth/msg04656.html

For guidance on FAX usage, get from http://linmodems.technion.ac.il/packages/  get faxing.tar.gz
It has samples for a modem using port /dev/ttySL0, which must be changed to match your modem's port.

Read Modem/DOCs/YourSystem.txt concerning other COMM channels: eth0
Which can interfere with Browser naviagation.

Don't worry about the following, it is for experts should trouble shooting be necessary.
==========================================================

Checking for modem support lines:
--------------------------------------
     /device/modem symbolic link:
slmodemd created symbolic link /dev/ttySL0:
     Within /etc/udev/ files:

     Within /etc/modprobe.conf files:
/etc/modprobe.d/alsa-base.conf:options snd-atiixp-modem index=-2
/etc/modprobe.d/alsa-base.conf:options snd-via82xx-modem index=-2
/etc/modprobe.d/blacklist-modem.conf:# Uncomment these entries in order to blacklist unwanted modem drivers
/etc/modprobe.d/blacklist-modem.conf:# blacklist snd-atiixp-modem
/etc/modprobe.d/blacklist-modem.conf:# blacklist snd-via82xx-modem
     Within any ancient /etc/devfs files:

     Within ancient kernel 2.4.n /etc/module.conf files:

--------- end modem support lines --------

まず、WVDIAL(ダイアラー) パッケージが導入されていないので、下記からダウンロードしてください。
http://linmodems.technion.ac.il/packages/wvdial-karmic.zip
ダウンロードしたら、
$ unzip wv*.zip
として、解凍して出来たフォルダに移動。
$ sudo dpkg -i *.deb
として、フォルダ内にあるパッケージをインストールします。

その後、下記より SLMODEMD.gcc4.4.tar.gz をダウンロード。
http://linmodems.technion.ac.il/packages/smartlink/SLMODEMD.gcc4.tar.gz
$ tar zxf SLMODEMD.gcc4.4.tar.gz
として、解凍して出来たフォルダに移動。
$ sudo ./slmodemd -c JAPAN --alsa -shw:0,1
端末から上記のように実行すると、
/dev/ttySL0 --> /dev/pts/N
と言う感じでリンクが張られ、ポートが作成されると思います。
最後の N は環境によって異なると思われます。

上記で挙動が変わらないでしょうか？

えぇと、とりあえず

# ./slmodemd -c JAPAN --alsa -s hw:0,1
SmartLink Soft Modem: version 2.9.11 Mar 13 2006 18:27:33
symbolic link `/dev/ttySL0' -> `/dev/pts/1' created.

とのことなので、モデムポートはきちんと作成されたようです。
私の所ではそこまでの操作で、kppp や gnome-ppp でダイヤルアップ接続出来ました。
※実際に私の使っているのは 82801DB/DBL/DBM なので、インストールするドライバや操作が微妙に異なるのですが、流れとしては同じです。

もし、これでダメなのならば、#9 で

Sorry.  You can retry the autodetection at any time by running "wvdialconf".
   (Or you can create /etc/wvdial.conf yourself.)

とのことなので、端末から wvdialconf コマンドを実行してください。
これはシリアルポートからモデムを探し出して設定ファイルを書き出すコマンドです。
下記は私の環境の実行例です。

petit@bp1-laptop:~$ sudo wvdialconf
Editing `/etc/wvdial.conf'.
Scanning your serial ports for a modem.

Modem Port Scan<*1>: Scanning ttySHSF0 first, /dev/modem is a link to it.
ttySHSF0<*1>: ATQ0 V1 E1 -- OK
ttySHSF0<*1>: ATQ0 V1 E1 Z -- OK
ttySHSF0<*1>: ATQ0 V1 E1 S0=0 -- OK
ttySHSF0<*1>: ATQ0 V1 E1 S0=0 &C1 -- OK
ttySHSF0<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 -- OK
ttySHSF0<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 -- OK
ttySHSF0<*1>: Modem Identifier: ATI -- 56000
ttySHSF0<*1>: Speed 4800: AT -- OK
ttySHSF0<*1>: Speed 9600: AT -- OK
ttySHSF0<*1>: Speed 19200: AT -- OK
ttySHSF0<*1>: Speed 38400: AT -- OK
ttySHSF0<*1>: Speed 57600: AT -- OK
ttySHSF0<*1>: Speed 115200: AT -- OK
ttySHSF0<*1>: Speed 230400: AT -- OK
ttySHSF0<*1>: Speed 460800: AT -- OK
ttySHSF0<*1>: Max speed is 460800; that should be safe.
ttySHSF0<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 -- OK
Modem Port Scan<*1>: S1   S2   S3   SHSF1 SHSF2 SHSF3 SHSF4 SHSF5
Modem Port Scan<*1>: SHSF6 SHSF7
WvModem<*1>: Cannot get information for serial port.
ttyUSB0<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
ttyUSB0<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
ttyUSB0<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.

Found a modem on /dev/ttySHSF0, using link /dev/modem in config.
Modem configuration written to /etc/wvdial.conf.
ttySHSF0<Info>: Speed 460800; init "ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0"

そして、下記がその設定内容です(私の場合)。

petit@bp1-laptop:~$ cat /etc/wvdial.conf

[Dialer Defaults]
Init1 = ATZ
Init2 = ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
Modem Type = Analog Modem
Baud = 460800
New PPPD = yes
Modem = /dev/modem
ISDN = 0
; Phone = <Target Phone Number>
; Password = <Your Password>
; Username = <Your Login Name>

上記の
; Phone = <Target Phone Number>
; Password = <Your Password>
; Username = <Your Login Name>
のところに、電話番号、パスワード、ログイン ID を記述して保存。
端末から、
$ wvdial
として、プロバイダのアクセスポイントなどに接続出来れば、まずは成功だと思います。