This is all totally untested, (at least by me), but Ned Konz, Randal L. Schwartz and Duane Maxwell suggested that it's quite sufficient to have a working telephone and a working soundcard. "All" you need to do is to get Squeak to produce the correct tones and to hold the telephone next to the speaker as it produces them!
This may be the only way to get this feature to work if you have a cable modem or no modem at all. It might also increase your street credibility if done in a public place with a laptop!
Unfortunately, I've no way to test this yet (until I get a soundcard that has a viable Linux driver), so I can't do much more than point people in the directions suggested by Ned, Randall and Duane.
The class you need to use in order to get this working is called AbstractSound.
From Squeak's World (red click) menu, open and then browser. Yellow click in the top lefthand pane of the browser and select find class..... type
AbstractSound and either
click on Accept or hit [Enter].
Here, I've clicked on the class tab (most examples are to be sound on the class side) and on utilities and dial:.
You can just select the text AbstractSound dial: '867-5309' (remember, don't select the double quotation marks or Squeak will interpret it as a comment and nothing will happen!) and hit [Alt] d and you should hear the number being dialled. You can now copy and paste that into a workspace and substitute a real number and hold the 'phone to the speaker.
If this works, you're well on the way to being able to re-write the code to diallup through your speaker. If not, you may find that the tones used in your country are not those used in Squeak (which do work, I'm assured, in the U.S.). You can probably find the equivalents for your country on the Internet.
I'll try and do some more work on this when my soundcard is sorted!