I had the same issue as you.
error 503 with warning like this :
Warning: 399 Server name "Unable to find a device handler for the request received on port
The issue come from on my gateway i have "server-group" with IP of sub pair.
But on my CUCM SIP trunkl, i have DEVICE POOL with other SUB pair for manage call of this trink.
When i update server-group with right ip adress. every wsorking good !!!
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