This question originally came up in our Slack community and the thread has been consolidated here for long-term reference.
How do I set the SIP port of an outbound trunk to something other than the default 5060?
Append the port to the address field in your trunk configuration:
"address": "<my-trunk-domain-name>:<optional port>"
The default is 5060 for TCP/UDP or 5061 for TLS. Add the port only if your provider uses a non-standard port.
See the documentation: