I currently have 2talk configured on Android with the built-in SIP client and getting poor battery life. I gather switching to TCP instead of UDP helps increase battery life because it changes to sending a keep-alive signal every 15min or so instead of every 30 sec or so for UDP. Is this correct with 2talk and Android native SIP?

However, for actual voice communication I gather UDP is better than TCP because UDP doesn't have the overhead of ensuring every packets arrives ok as resent packets would be delayed and therefore of no use. So if I configure the Android native SIP to use TCP is this just for establishing a call but the call itself does it still use UDP?