edit: This was premature. See my reply post below I got the Android SIP phone "SipDroid" (v1.5.2 beta) working with 3CX. It works both behind our local WiFI router (NAT) to our public LAN (3CX is on the public LAN) and on 3G. The critical items are: Supports Re-Invite must be OFF STUN is required, using stun.3cx.com/3478 PBX Delivers Audio can be OFF Before doing the above, I had problems with audio when calling from the Android phone to another extension (as discussed elsewhere on this forum). Now it works fine. This is a good phone, but it does have a few annoyances. The most useful feature is its ability to trap numbers dialed into the regular Phone app and route them to SipDroid and thus to the PBX. I have mine set to require appending a '+' to a dialed number to force it through the PBX, but you can set it so that calls go through the PBX by default, and use the '+' to force the call through the mobile network instead. The Transfer feature works strangely, but it may be me... I'll research this and post back if I find anything. The other thing you'll notice is on the 3CX admin "Phones" display, SipDroid will appear as a phone multiple times with different (dynamic?) port numbers. This appears to be harmless. Video does not appear at al, and may require their pbxes.org server?? If you need more set up info, ask away and I'll try to help. I decided to go this route instead of posting a detailed setup procedure here as I am a bit time-limited at the moment! For info see the Google Code home page for SipDroid and the FAQ in the Wiki there.