jtmarcot,
Here is another suggestion for a European server that you can try for the incoming calls from IPKall. You can try incoming calls for free if you signup and register to ippi's free account. Get an account at ippi (
http://www.ippi.com/) which has a server in Paris. They have an English language web page. When you signup they give you a sip uri for calls coming to you. If it looks good incoming you can prepay outgoing calls if you wish but you don't have to.
Thru a test, I setup my IPKall number to forward to my ippi sip uri. I verified that I can call my number at IPKall using G.711u, have IPKall forward the call to my ippi account, and ippi does transcode to G.729 when sending the call to my registered OBi100.
I would configure your OBi SPx sip port (X_UserAgentPort) to something non-standard for your (the OBi's) sip signalling side of the call. Ippi does use port 5060 for their side of the call.