Actually it's all about the quality of your connection...

May I ask if you have had experience setting up SIP/VOIP on another device. Do you have another SIP capable device where you can try your GV credentials? If it works on that other device and not your N9 then there must be something wrong with your n9. If it doesn't work for both, then your WiFi isn't prioritizing your VOIP. (i.e. If you're connected on a wifi network that's also serving a client who's using the network heavily while someone's trying to call your VOIP number, chances are, the call will not get through, or even if it does, you'll get a choppy reception/delayed voice or worse, drop calls.).