not sure if you can bar internal calls but you can make these changes the CLS
The seven Class of Service types are:-
UNR - Unrestricted The extension user can dial anywhere they want - literally without restriction.
FRE - Fully Restricted No exchange line access. The extension user can make internal calls, can use privately wired (Tie) lines, however they can have calls using exchange lines transferred to them from another extension user.
FR1 - Fully Restricted 1 Same as FRE, but calls using exchange lines cannot be transferred to them.
FR2 - Fully Restricted 2 This restricts the extension to internal calls only.
SRE - Semi-restricted Allowed outgoing external calls via the attendant console only. Can receive external incoming calls.
TLD - Toll Denied Allowed outgoing calls to certain destinations only. The Meridian 1 monitors the digits that are dialled to determine if the call is to be allowed. This process is called New Flexible Code Restriction.
CTD - Conditionally Toll Like TLD, CTD checks the digits dialled, however CTD Denied only applies when a PSTN route is chosen as an alternative to a private network call.
Try That