I am searching for a way to make a point-to-point VoIP call between two Android devices without having any intermediate server (SIP server) or an Internet connection. I just have an access point (or multiple access point connect via LAN) and I want to make VoIP calls over Wi-Fi only. Is this possible?
You can use CSipSimple to add a local SIP account on both phones without having the need for an intermediate SIP proxy (or server).
Add a general 'Local' account. You only have to provide a SIP username for it.
When that is done on both phones you can call the other phone:
Click on the green marked spot in the left image to navigate to the SIP input activity :
Like one device has uSipServer, started Wi-Fi AP (in "Tethering" menu) and SipDroid connected to email@example.com. Other device connects to Wi-Fi and use SIP account firstname.lastname@example.org and calls to "test1".
Update: Have just tested that this configuration actually works:
I tried other SIP clients (like CSipSimple and Linphone), but they refuse to work on the first device when no "normal" network is configured. And SipDroid (which is fortunately dumb enough to overlook network "absence") is not codec-compatible with everything else I tried.
Note that if there are other network connections present, SipDroid may confuse them and send the wrong IP to it's peer resulting in "one way" phone. Test with only Wi-Fi hotstop/Wi-Fi client enabled.