O2

So I completed my contract payments for my tablet last year and I've just now realized I'm still paying for the SIM that came with it even though I'm not using it, how do I cancel it?

So I recently just paid off my device plan and upgraded to a new phone, the new phone comes with a new sim with a new airtime plan, does that mean I'll have a new number? If so how would I go about keeping the old number but having a new airtime plan...