Softphones can make calls internaly successfully.
When trying to establish a call from one of the Softphones to an external connection over the Trunks, this is rejected with “service unavailable” message.
What should be the settings in the FreePBX and the SBC for the trunk between them?
After some troubleshooting, both outgoing and incoming calls are working.
Especially the incoming external call issue took some time to solve, as the configuration to make this work, is different than documented in the FlowRoute pdf linked in the opening post.
On incoming external calls over the SBC , the external call leg between ITSP and SBC, the callee is simply the dialed number.
Following the documented call routing plan for incoming external calls, on the internal call leg between SBC and FreePBX, this callee is replaced by the registration account used for registering the SBC with the trunk towards the ITSP.
Reason for this replacement is the Call routing plan on the external call leg in the SBC, as pictured here: