I wanna use a sip extension on my cell phone that it is connected to internet by GPRS, can I?

Your Phone can register SIP extension (for instance with Asterisk PBX) but dont expect it to be used as Extension. Data Rate (Specially Uploading) wont allow a normal call to take place, on3G networks it should work.

It Is possible but i don't think that GPRS can garenty a good voice quality.

In3G you can have a good connexion.

Calls sip should have25 Ko in Codec g729 et64Ko in G711.

 

What is your Mobile phone, and What do you use for SIP extension?

It is Sony Ericsson Xperia X8.
I wanna connect my cell phone to the VOIP network when i connected to the internet by GPRS

Connect to your local network through VPN, you'll be able to register/use your sip client

if you own the sip server you can use it but you need to assign static ip for you server