Can VoIP work on a GPRS-enabled mobile phone?

Is it possible to use VoIP on GPRS-enabled mobile phones?

Technically, yes -- if you are talking about the cell modem connection to the Internet. However, many providers restrict voice packets because of revenue competition with the minute-based voice plans. There are also new hybrid phones that will switch between 802.11 and GPRS -- but again, the usage for voice is restricted unless you have a plan that permits the conversations.