Racer, I agree that it doesn't seem to be server specific.

It's hard to determine what the cause is.

Some of my larger clients do thousands of calls per month, with no issues. So it doesn't seem to be a global issue.

I suspect it is some kind of port timeout issue on certain routers.

If you have an ATA or IP phone, be sure to change the registration timeout, aka server timeout, or whatever it is called in your device, to something other than the usual 3600 second default. It needs to be more like 300 seconds. That solves the random inbound call failure most of the time.

Setting up port forwarding also helps, if you haven't done that.

And make sure you don't have a double NAT (one router/firewall behind another router/firewall).

My problem, is that I've already done all the usual stuff... eliminated double nat, setup port forwarding, corrected server timeout, etc. But I still get some random failovers to voicemail on certain user accounts. These accounts are usually numbers that had some issue during porting in, so it just makes me wonder if there's some kind of provisioning issue going on with these new numbers, so it doesn't affect older accounts.

But it sounds like I'm the only one with this specific issue, so my guess is it must be pretty "user specific", such as some particular carrier glitch, or maybe just the router the end users have.

I'll be doing some remote sessions with these users, so maybe I'll spot something.

Thanks for the info.