DSL MODEM WOES

A guy I work with has hit me with this question, I don't think it is doable but you guys might know for sure. He and his neibor have droped sprints DSL service and still have the Zyxel modem that came with it.

Now they are looking at a new DSL provider and want to try useing their old modem to save buying one at the steep price from the new provider. Is this doable or is the modem locked for sprints sevice only. Can I telnet in to it to change the config for it to work.

FYI i think it is a Zyxel Prestege 645