Which is the most acceptable way to create a VoIP/Instant messaging app?

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http://stackoverflow.com – I want to create my own VoIP / Instant messaging app. I haven't used android for something even remotely similar. Searching the web i haven't found an acceptable answer. I have looked at SIP protocol but i don't know if it is very reliable with instant messaging. I would appreciate any hint on which direction i should move. Thanks... (HowTos)