No, you don't buy the sim, when you buy the phone either the shop you buy it will insert a sim and sort the number for you, all done before you get home or else, if they are an independent company like carphone warehouse or something, they will give you a sim, tell you to ring your service provider, tell them you have a new phone, new sim but you need your own number and it will get sorted for you, it's very easy transfer over and they do the work, not you in most cases, the shop will tell you in a much clearer way but the jist is, you'll keep your number