I have six extensions.  When an outside caller calls into the office, all phones ring and whoever is available can answer same and then transfer as needed.  If no one is available the call is routed to an  live answering service.  

This all works fine.  However, in many instances one extension may need to call another extension.  For instance I need to call one of my assistants. If they pick up no problem.   However, if they dont pick up the call is treated like a missed call and routed to the live answering service.  I dont need this to happen as this is just an internal call.  

 Is there a way to distinguish between an external call and an internal call so that if a call one of the extension and there is no answer it just goes to voice mail or there is some other announcement.