Outbound call fails when inbound call active

I am having outbound calls failing when an inbound call is active. My setup is as follows:

FreePBX 12
Asterisk 11
Server is virtualized in house
Behind a firewall with public IP assigned to FreePBX server

I have tried turning NAT off in both Advanced Settings and at the extension with no change in behavior.

Multiple outbound calls CAN occur successfully simultaneously.

I don’t know where to go from here.

Thanks in advance -

Can you tell us a bit more about the trunking you are using?

I am using a sip trunk from century link that is tied to an IP address. I have a rule in my firewall that opens up the sip and rtp traffic to FreePBX server. The century link sip trunk allows 20 sessions.