When will Voxalot return "Congestion" to a provider?

[dnuser]

I'm working to setup Voxalot, using Vitelity for my DID. Almost everything seems to be working well, but about half of my incoming calls are failing with a busy signal. Vitelity reports "We received 'CONGESTION' when attempting to route the call to your server or device."

To help me troubleshoot, it would help to know under what conditions Voxalot will return a "Congestion" response to the provider.

Here's what I've figured out so far in troubleshooting:

- This does not appear to be related to my ATA, or my connection to Voxalot. When I disconnect my ATA, half of the calls fail with "congestion", the other half go to Voxalot voicemail.

- Voxalot seems to be registered to my Vitelity account. When I turn off "SIP Register" at Voxalot, all calls fail with "CHANUNAVAIL" from Vitelity (the same error as when I route calls to an account that is not registered.

- I've tried several different NAT settings at Vitelity, and have tried with "Symmetric NAT" both on and off at Voxalot. The results don't change.

- The successes and failures seem to be grouped together... incoming calls work for a few minutes, then stop working for a few minutes, then start working again.

Any help would be appreciated!