FreePBX trunk to UCM6100 says busy but extension rings

I have a trunk connecting FreePBX with a Grandstream UCM6100.
The UCM can place calls to extensions on FreePBX. FreePBX calls to the UCM says the number is not in service but the extension on the UCM will ring until timeout.

Not sure what side is causing this. Any ideas?


Have you ever figured this out? I also want to use a Grandstream UCM 6116 for it’s FXO capabilities, to connect to an analogue Panasonic KX-TD100D switchboard.