The DT Client has specific options for internal calls

The following settings are global, so they apply to the whole team. If you need users with different options then you need to create a separate Settings Team.

In order to detect internal calls, be sure to set the extension length. Set it to the longer length you might have. For example, if you have 4 and five digit extensions, set it to 5.
Setting: Provider Configuration > Internal Extension Length

If you have mandatory fields on your Phone Call form, or whatever form is popped by the DT Client for inbound calls, you might want to turn off validation for internal calls.
Setting: Advanced Configuration > Validate Internal

If you want to stop a CRM Phone Call being created for internal calls, you can do this.
Setting: Advanced Configuration > Internal Phonecalls

If you want to take your internal calls on your twinned deskphone, you can set the DT Client to ignore internal calls completely.
Setting: Advanced Configuration > Ignore Internal