SIP to SIP calls (internal)

I have GW1600-16G Gateway. It is configured as a standalone gateway with 2 SIP endpoints.
However, SIP to SIP calls are not working/ I just can't call to the other SIP client and what is more frustrating - both clients can't be seen by each other. All GSM calls work, as well as SMS do. I have tried to different softphones; Jitsi and Zoiper, and both seem to work fine with GSM calls, but each time I add the other SIP client to adress book or try to dial him - I get an error (stating that this account is not found on server).
I wonder if there is anything wrong with my configuration? Maybe I should perform an advanced Asterisk configuration to enable this feature?
PS: If there is no workaround, I wonder what should I do to enable calls transfer from one client to the other via softphone.
(If you try to reproduce this bug, please try Jitsi softphone first, as it appears to work better with the gateway)

Contact me via skype and prepare remote access.
I will help you figure it out.