That seem odd... modems
anyways you can:
1. open the modems with a terminal program, and set it up to wait for calls. Then remote user use his terminal program to send you files.
2. setup pppd and ...? - I forgot the program name I used in an ISP a long time ago. IIRC it's name started with "p".
Though if your modems are soft/win modems, it will be harder or even impossible.