Hello

My carrier is BouyguesTelecom in France.

Before making the decision of either getting a sponsored 8520 for 49€ or buying it direct for 250€, I need to know if BlackBerry or a carrier can ensure that the user of a wifi-capable smartphone cannot install an SIP client so as to bypass the carrier and make free phone calls through the Net.

Incidently, is the wifi feature of the 8520 good enough for VoIP, in terms of performance + battery use?

Thank you for any tip.

AFAIK there is no native SIP client for BlackBerry available yet.

Please correct me if I am wrong.

You are not wrong.
At this point, there are no SIP clients (or Skype for the BB).