I presume you know the number to dial, this is normaly easy to find anyway. I'm not sure entirely what tool you could use but you may need to know what authentication is used. Heres how to find that out:
-Open up hyper terminal (hypertrm.exe?)
-Connect dirctly to your modem (try Com1)
-Once connected type the following:
ATZ -- Gets modems attention
ATM1L1 -- Turns on the speaker
ATDT01234567890 -- Tone Dials the number

-After this one of two things should happen, if you are presented with a request for use/pass then they use PAP (password authentication protocol), if you get a load of characters thrown at you they use CHAP (Chalenge Handshake Authentication Protocol).

I know thats not much but I hope it helps.