If it is an existing monthly subscription account, e.g. surftime, anytime. Then you will not be able to set it up from your house.
ISP's hand shakes, when one computer communicates with an other over the net, need 3 basic tests.
1) is the phone number
2) is the user name
3) is the password
All 3 must match.
Sometimes a fourth component can cause a problem and that is the computers unique I.D. number.
You would do better trying to set up the internet from the users address.