By default, the OBi routes all incoming calls on any trunk to the PHONE Port.
Are you wanting to prevent the possibility of any outgoing calls on any/all trunks?
It's an unusal request, but the PHONE Port DigitMap could be pruned to accomplish this goal.
The following would limit the PHONE Port to connecting to the configuration menu only:
Physical Interfaces -> PHONE Port -> DigitMap : (***)
