Your posting history suggests this issue has dragged on since last March! Unfortunately, VM sends out tech's with widely-varying levels of ability and knowledge. Some past topics, about phone rewiring post-switchover, have described multiple visits from different VM tech's being required until the 'right' one turned up to get all items of equipment working for the customer.
What sort of age is the external bell unit that you want to connect?
Telephone equipment from the early 2000's, and before and particularly BT equipment, might need a 'ringing capacitor' in the circuit and a bell wire to be connected throughout the telephone cabling in order for the device to ring. Modern equipment does not use these and they are not present in the default arrangement when the VM phone line is provided from the VM hub.
