I think that he already has a phone line connected to his modem (he's using dial up). I'm guessing that he wants caller id while he is online. I know that I get caller ID on my phone when I'm on a call. So I can screen while the phone is in use.
My guess would be that Verizon and at&t do not have any software to do this, but if you search the net enough, then you might find something.
I searched for "Caller id" on
Free Software Downloads and Software Reviews - Download.com and found 99 hits. So there is stuff out there for you to check out.