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You can.

You set the phone as a Limited Extension - and in there you can state no external calling.

On the phone itself - depending on the model, you can also program the DSSKeys to show which extensions you allow them to dial.

For us, I have a yealink set with our Billing, Nursing, and Pharmacy extensions - and the client just needs to pick up the handset and press one of the buttons.

If you have a yealink or polycom - let me know if you need any screen shots to help set up.

Have fun.

There is no setting for this. However, you may create a User extension with no Digital Line but a direct number only. It can only call internal numbers through the app.