Asterisk and phones through VPN

I have a main Asterisk server located at our downtown office. We also have several locations throughout the city that have Cisco 7960’s. To overcome the NAT problems, I have set up VPN’s using OpenVPN. This way, everybody is local to the server and has no trouble calling in and out.

The VPN is in UDP mode and the latency is at most 35ms between any location and the server.

We also have 3 Voip providers that we use depending on where we call.

The problem is that on one of these providers we get choppy sound. It’s a provider for calling Romania and my gut feeling is that there is a problem with our tunnel.

Here’s what I have so far:

  1. If you call directly from any phone within our system towards Romania you get choppy sound quality

  2. I set up the same provider at extension 6900 through DISA. If you dial from the PSTN to our main office #, dial 6900 and then your romanian phone number, the call will be crystal clear

  3. If I dial 6900 from any inside phone, I get a dial tone but it’s very choppy

  4. If I dial voicemail or any other north american phone number I get very good quality.

I’m stumped. If anybody has any clues…I’d greatly appreciate it!