Based upon the very limited information you've provided, I'm going to guess that the problem is in your routing between subnets. Not all routers do a good job, and many screw up routing of SIP. Same with VPNs. Have you tried a different router? Have you tried a different VPN? I use OpenVPN with Vyatta and it works great. I migrated from an expensive Cisco solution that was terrible.
Also, I continue to believe that PJSIP is not ready for primetime, and that you should stick with Chan_sip for now.