Another silly question and a few thoughts...

1. I note you use authenticated proxy. Did you set up the Grandstream to authenticate?

2. I agree with Frederik, that either STUN or ICE is required. I use STUN and it works like a dream. However, VOIPO does not advertise supporting STUN so you need either setting it up using a public STUN server - anyone will do - or find out how you set up STUN (or ICE) with VOIPO.

3. I notice from VOIPO's installation instructions that the Grandstream appears to be provisioned over the net. The Proxy Server may get in the way of the provisioning process. I once had that problem. Explaining the issue to my VOIP provider enabled them to resolve it from their end.

Either way, opening ports or otherwise exposing the Grandstream directly to the internet should be avoided at all cost. If you can't get the VOIPO service to work, I suggest dropping them in favor of buying your own voip box (or use your old Obihai 202 if its open for configuration) and then using one of the many voip services that do not wall you into a closed garden. That's what I do and have done for the last 10 years. It works like a dream and costs me about $60 a year for two incoming phone numbers (US and Denmark respectively) and unlimited outgoing national and international calls. The wife loves it!

Cheers,

Peter