I have seen the issue you are talking about with Versature sip trunks. In that case the timer on the versatile side was set to 60 minutes. Way too long, but not the only issue.
It that case the trunk would time out, and have to re-register. Unfortunately the DNS was taking too long and the a time out would be sent.
The trunk would eventually come back up on it's own, but in the meantime the users would get waiting for line presented on the phone... outbound of coarse. Assuming a busy signal on inbound.
One "bad" workaround was to manually take down and bring back up the trunk in SSA. This requires eyes on the trunk at all times. This helped keep the business running during troubleshooting.
Is it possible for you to use IP addresses instead of domains? If you getting the same DNS timeouts I had, that help.
Check with Vonage on what their timers are set to. Should to be more than 5 minutes. 60 may be fine for residential service, not adequate for business.
At the end of the day, I switched to DIDww.com sip trunks. No problems since.