I'm having similar issues as well (caller gets number is disconnected),
a few times on outbound, I get fast busy when dialing out.

I opened a TT (HBG-874105) and support is having me set up port
forwarding inside my pfsense box.

Somehow, I screwed that up (when I set the ATA to a static, it
no longer registers I can't get back into the ata - but that's a seperate
issue I hope support can help me with ... :-)