Heh, that was the first question I ever asked on a usenet security board when I got interested in the internet to a more technical level. (and damn what a flaming I got, im still putting honey on the wounds)

Bascially if we think of a 56k modem for easy of explaining (ADSL, Cable and other means work the same to an extent), if when your connected to the internet on your 56k modem, you lift your telephone reciever and have a listern you'll hear the "computer talking", this is modulated TCP/IP packets, the modems job is to modulate and demodulate the packets to "talk" to your ISP. Where as you are dialing into there service using the phones all the information is routed in this modulated signal. So you can see that the IP is seen in this ISP connection, which is placing your computer on there network, becuase thats basically that you are, a computer on the ISP's LAN, kinda... (dont pull me apart here im trying to make things easy to understand, whilst prehaps not technically correct)

With a normal telephone your PSTN (public switched telephone nework) routes the calls based on the number you have dialed, this information is contained within the noises that the modem makes when it dials the ISP.

So no, the actaully hardware level telephone number isnt contained within the details that you as an attacker or otherwise have access to.

Hope that makes sense.

i2c

ps - heres some links that might help you understand

http://computer.howstuffworks.com/modem.htm -- about modems.
http://electronics.howstuffworks.com/telephone.htm -- about telephones and there networks


edit - the only way I can think to get the telephone number would be to drop a program on that dials your number and you have called ID on your phone, that would give you the number, although its illegal...