[jitsi-dev] Jitsi failed to establish connection


I've download the Jitsi open source application for android devices, and I am trying it inside the office with my friends.An xmpp server was created in order to link the clients in order to chat and call.Chatting is working fine, but the problem occurs when 1 phone is calling another phone which is on a different network from the first. What I means by that is the following:if user A called user B, and user A and B are the same network(same wifi router or same 3g operator....) the call is established and we can talk to each other.if user A (on wifi) is calling user B(on 3g), the call failed with the following error ( Could not establish connection : ICE failed )
Any idea why it's giving me this error when we are on different network ? Is there any plugin I need to install on the xmpp server in order to solve this issue ? Is there any settings that should be changed from the client side ? I am stuck here, and I need to solve this problem, so any help would be appreciated.
Thanks in advance,
Elias Nabhan