I'd also like to know how to do this, and I'm shocked that the speex codec is not supported by the native dialer. g711.u (Ulaw) is the default codec, and is simply not usable while driving. GSM works well over 3g/4g on my GSM Galaxy Nexus over T-Mobile, but it sounds like you're talking through a tin can. g711.u is great over WiFI, but WiFI is not always available. Using the integrated SIP phone is much better than using CSipSimple and sipDroid. When CSipSimple works, it's great. Speex sounds great over 3g, but CSipSimple is very tempermental.