When people call me on my BT mobile and I am on a call they get the normal ringing tone and then voicemail.
I would prefer this to an engaged tone so they know I'm on a call. Is this possible?
Would anyone else prefer the engaged tone?
That is a setting on your phone and has nothing to do with BT mobile.
Look at the call forwarding options and select the one that corresponds to what happens when you are busy. You will find the voicemail number there and there should be an option to disable it.
Look for call waiting in your handset. If that is on, which it sounds like it is, it will let another call come through even if you're currently on another call. If you disable it then this wouldn't happen, and people would go straight to your voicemail instead.