I purchased a number of Vigorphone 350 handsets and a Draytek 2820 IPBX for a small installation. The Draytek 2820 IPBX did not work with my POTS line until the latest firmware revision so I was forced to purchase a Grandstream GXE 5024. I used this in place of the Draytek 2820 IPBX and it all worked fine until the latest 06 version of the firmware for the Vigorphone 350. Now the Vigorphone 350 only work for internal calls and not external calls. I also tried a Linksys 3102 but that did not work correctly either.

I think Draytek still have quite a bit of work to do on the Draytek 2820 and 3510 IPBX systems before they work well with POTS lines.

I have done a bit more testing and this is what I find. If a Grandstream 2020 VoIP phone calls a Vigorphone 350 (interal call) everything works. If the Vigorphone 350 calls the Grandstream 2020 VoIP phone (internal call) the microphone feeds straight back into the earpiece instead of going to the Grandstream 2020 VoIP phone. The Grandstream 2020 VoIP phone microphone works correctly and the sound arrives at the Vigorphone 350. You get exactly the same if you use 2 x Vigorphone 350's. External calls (incoming or outgoing) do not work in either direction.