CSipSimple is an open source Android softphone. It only supports audio and messaging, not (reliably) video yet. It has good support for SRTP and ZRTP encryption, uses SIP over TLS and supports ICE and TURN for NAT busting.

If your account fails to register with the above settings, check the global settings for "Network". Open the global menu (in the lower right corner) and choose "Settings":

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