Probably you have tried this already, but it looks like LINUXMCE firewall is blocking incoming traffic (in your case, from your phone to the internal network). Also, it could be that you don't have your modem/router VoIP ports redirected to your core.
Check firewall rules.
Be careful when opening VoIP ports to the exterior, default passwords are not that secure, so a malicious guy could connect to your network and make long distance calls from it.
I don't know if there is an android port of openvpn, but you should google it. (also OpenVPN is already implemented in LinuxMCE, so probably you would only need to change some server options, like making it Server VS Roadwarrior, change passwords, etc...).
Tell me if it was the firewall thing.
P.S. It could also be asterisk related (local encapsulation in SIP packages), see post 2 in this link for more info: http://forums.whirlpool.net.au/forum-replies-archive.cfm/1230714.html