NAT tables on mobile networks

I’ve been trying to configure csipsimple and sipdroid to be as effient as possible when using my mobile phone. I want to be available on my VoIP network provided by www.pbxes.com but I don’t want to send unnecessary keep alives and registrations for example which would not be great for battery life.. I was curious to find out which was the best way. Obviously, the longer the timeout the better, but what’s the maximum? This will depend on which network you’re on, but I’m on GiffGaff (o2) in the UK and found that a 4 min keep-alive works well. I also have STUN enabled, and have also enabled “wake phone on keep alive” in csipsimple. Compared to the default of 90s, this should significantly reduce the power drain.