I have a test system set up: FusionPBX 5.3.8, FreeSwitch 1.10.12. I have a Bulkvs gateway, a North American outbound route, one inbound DID pointed at a ring group and one extension. I have not created a second domain for extensions as I would on a production system. Inbound and outbound calls work from the extension.
Same setup as yours except using registration with Bulkvs. Watching sngrep, the call comes in and rings the extension. No sign of an outgoing call to the cell phone in the sngrep output. CDR shows call failed codec L16/L16.
Likewise, if I call the ring group and answer on the extension, I cannot manually transfer the call to the cell phone because my trunk to trunk transfer does not work. If I dial the cell phone directly from the extension, the call goes through.
Also, if I forward the ring group to the cell phone number, an incoming outside call will ring the cell phone (bypassing members of the ring group.)
If I dial the ring group (9000) from the extension, it does not ring the cell phone.