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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.