I use CSipSimple on Android with VoipBuster as SIP provider for outgoing calls - this works really well. However, you should be aware that unlike with Skype th

I use CSipSimple on Android with VoipBuster as SIP provider for outgoing calls - this works really well. However, you should be aware that unlike with Skype the SIP packets between your phone and your provider aren't encrypted. So if you use this on a public WiFi, everyone with sufficient skills would be able to listen to your call.