HowTO set Outbound route - ext. MobileNumber in a ringroup

Hello FreePBX Forum !

Our Setup is easy to explain.
We have setup for all extensions special Outbound routes.
Example Ext. 300 CallerID in OutboundRouteA = 300 so it uses explicity this Outboundroute =A. Perfect.
Now we have setup a “Ringgroup” to call an external “MobilNumber” The Ringgroup Extension is 600.

All externalcalls which where generated by the “following ringgroup” will not be routed “OUTSIDE”.

Ringgroup601 contains
Extension 100
Extension 101
Extern Nr 123456789#
Extern Nr 456788#

How to Setup THE outbound ROUTE to force a specific Ringgroup use this?

I would appreciate any help / tips / links to docs.

thanks a lot in advance for your help.