Forward calls to mobile phones from Time Conditions total silent when Ring Group calls work fine

I have a Time Condition set up so that on Monday, calls to an Inbound Route get directed to a mobile (cell) phone.

I have set up an extension to the mobile phone. Dialling 1604 calls the mobile fine and you can talk to the person on the other end.

I have added the extension to the mobile phone to a ring group. Dialling 1600 will call extensions 1601 to 1604 and if the call is answered on the mobile phone then you can talk fine.

I have added an inbound route for the number in question. If I set the inbound route to call the extension (1604) or to call the ring group (1600) then you can answer it with the mobile and talk fine.

If I set the Time Condition to call an internal extension (1601) or the ring group (1600) and set the inbound route to trigger the Time Condition then if you answer it on the internal extension it works fine. However, if I set the Time Condition to call the mobile (1604) or the ring group (1600) and dial the inbound route then the mobile rings fine and the call can be answered but you cannot hear anything at either end.

Have I hit a bug? Have I done something wrong?