I have a teleworker using a SPA525G2 with the latest firmware to our UC540.
He is able to connect via other ISP's both wired and wireless.
When he attaches his phone to the internet via his Verizon FIOS ActionTech MI242WR-Gen2 modem/router, and then launches the VPN, everything appears normal.
After he makes a call, he can hear the first few seconds of audio once the person answers. After that, its almost like the router is interpreting incoming packets as a DOS attack and after a period, the phone displays "network error" and reboots.
The person the teleworker calls can hear the teleworker the whole time.
When the teleworker phone does reboot. The teleworker phone sometimes crashes the UC540 as well.
The teleworker is on 192.168.1.1 (a different subnet than the UC540).
The teleworker has set his IP phone at 192.168.1.10 as the DMZ in his router.
All other rules and firewall settings have been disabled.