I am wanting to know whether I could use my current on Account sim card in a new phone (contracted to a different number) and have another number in my current phone. I'm also on a 24 month contract.
You will be able to use any sim card in your phone and your sim card in any phone. The sim card is not restricted to one phone. However if it is from a different service provider, then its best to check if the phone is network locked.