Group call queue doesn't route call to numbers in call handling.

We have a group call queue on extension 2. What is supposed to happen is that the call handling means the call first goes to the user at extension 18 and, if they don't answer, go to the user at extension 16; if extension 16 doesn't answer, it should go to voicemail. However, once the client chooses extension 2, the phone never rings for extensions 18 and 16; the call rings for the allotted amount of time on the client side then goes to voicemail. The users at extensions and 18 and 16 get no notifications other than that a voicemail has been left. How do I fix this?