I created the SIP trunk between Diguim switchvox and DMA. The SIP trunk is UP. The call from IP phone registered to digium to VMR on the RMX registered to DMA is ok and the phone able to join the conference.
RMX prefix 72, SIP trunk prefix 77
when The phone dials 77721001 it connects to the RMX normally.
When trying dial out the IP phone (772009) from the RMX the call failed.
So it looks like the DMA has a dial rule for the SIP trunk to the Diguim switchvox. I would make sure the IP address of the Diguim switchvox is included in a subnet configured in the site topology of a site. If you don't, you run the risk of the DMA trying to route the call through a firewall or some other SBC.