Vista - dial up computer with xp from a computer visita home premium

I would like to be able to dial up my computer at the office and use remote
desktop. I do not have a network! Just a modem to modem connection. I have
set up the window xp pro office computer to access incoming calls. My home
computer as been config to dial to office with computer name, user name
password. The computer dials the modem picks up and 60 -70 seconds later the
calls fails. I have made adjustment to the windows firewall to allow for the
incoming call. Any ideas, I should also tells you the phone line at the
house is IP and the office analog.