SIP Trunk with 4 Channels only accepting 1 inbound call

I am experiencing issues with my FreePBX installation/configuration. I have successfully configured inbound and outbound calling. Multiple outbound calls at once work without issue. A single inbound call is fine, however when receiving multiple inbound calls, the first call connects successfully. Whilst subsequent calls do not and I receive a busy tone.

I have two inbound dialling routes, I test by calling one of them (telephone number ending in 59) then try to dial the other one (ending in 50). I get the expected result with the first call, but the second is always returned as user busy tone.

I have spoken to my SIP provider and they have assured me that its configured correctly on their end. I’m just stumped with possible issues on my end.

To avoid flooding this with loads of config logs when not necessary, please let me know what you may need.

I’ve read through the documentation, and many threads on this forum in search of the answer, but it has alluded me thus far, any help would be greatly appreciated.