Numbers are chosen on the Cloud Voice order, or left until the seats are live. Nothing on that step is a monthly line rental.
New numbers
You can order a new UK geographic number or DDI, or a new 03 number. Each is a one-off fee, shown at checkout, ex VAT.
Moving a number you already have
Geographic numbers can be ported as a group. A single 03 number can be ported, as can a sequential block of up to 10, or a sequential block of up to 100. Those are one-off fees too.
The wholesale operator sets the process and the timing. A specific porting date is not guaranteed. You remain responsible for any charges with your current provider until the port completes.
Later
You can leave the number unset and arrange a new number or a port after the seats are live.