Best answer by Hikari
If you would like to just change the phone number of your existing contract then you can contact iD Mobile by Live Chat or by phone on 7777 and request it. Bare in mind this service will be charged £25 one of fee.
If you would like to order new number on additional sim card you can do it after paying 3-6 bills. Then simply head to idmobile.co.uk or visiti your local Carphone to search for new deals.