I have the HT-502 and don't have any problems. I do realize my network setup is different than most home networks. My HT is behind my router like most but I have no ports forwarded and the router SPI NAT firewall is enabled. I let a STUN server negotiate the necessary ports for VOIP.

Have you let VOIPo configure you're router for you. They are some very talented folks, Brandon is superb at troubleshooting. Let them take a look at your router settings, if you haven't already.