route fax extension on trunk

[CipherTV]

I have setup a fax ATA grandstream ht801 using a SBC to connect to my 3cx server. It works when I use a normal extension as the default route on the trunk, but when i try to use a fax extension with the same DID it does not ring. I have tried to change the trunk default route to other options but does not make a difference. Is there a setting to use the fax extension as the default route?