The internal modem in the Thinkpad T42P is one of those softmodem thingies where all the smart stuff happens in the driver and very little happens in the modem itself. These things strike fear into the heart of Linux driver creators because they can be um, quite opaque in terms of documentation and barren in terms of manufacturer support/interest in supporting Linux.
It is a straightforward matter of market segmentation really. These things are built to be stacked high and sold cheap, bundled with pre-configured PCs in lots of 10,000,000 units. In a word, they are designed to work with Windows.
Anyway, to make a long story even longer, many of these soft modems are variations on a theme known as the Conexant Chipset. (Formerly Rockwell for us old-timers). Consequently, a Linux driver that groks Conexant Chipset covers a lot of ground. Linuxant have a slow-ware driver known as the "HSF Driver" that works just fine with Ubuntu on the Thinkpad T42P. You can get it here.
The file I grabbed was hsfmodem_7.18.00.05full_k2.6.10_5_386_ubuntu_i386.deb.zip
The install instructions are here http://www.linuxant.com/drivers/hsf/install.php.
The install proceeded with no problems producing a bright shiny /dev/ttySHSF0 and a proud symbolic link /dev/modem.
I used minicom to send the modem equivalent of ping - the "AT" command and got back a resounding "OK".
From there I followed the pppd tutorial by Tony Mobily from Linux Journal:
part 1 and part 2.
Note 1: Its very painful navigating a shopping-cart dialog at 14k :-/
Note 2: It is good for the soul to get down into the bowels of Linux from time to time. The beauty of open source is that the bowels are there to be seen, in all their fierce beauty, when required.
There are no signs saying "STOP. YOU ARE NOT ENTITLED TO GROK STUFF FROM HERE DOWN".
I like that.