The requirement for the Mobile Client to work is that the device must have access to the Internet.
Normally phones can access the Internet via GPRS, 3G or WiFi. The Mobile Client software doesn't care which method is available (it might not even be able to tell), as long as one is available.
Just relying in WiFi isn't a good idea IMHO, if your device goes out of WiFi coverage (i.e. outside the office or home) and there is no GPRS/3G service available, the Mobile Client won't funtion. That said, incoming calls via Mobile Extension should continue unaffected (I believe, but not tested).