Still hear dial tone when calling custom extension that calls external number

I have a custom extension configured to dial an outside line (my mobile phone). If I route a DID to this extension and call the DID from an external number, the call goes to the mobile phone as expected. This works beautifully. However, if I call this extension from an internal extension, the call completes but I can still hear the dial tone. I hear both the dial tone and the called party talking.

Can anyone offer assistance?