This is going to depend on whether you are on contract or if you pay for your phone as you use it. If you are on contract and there is a problem with the sim that you have, you are going to have to call the service provider that you are with and they will be able to help you fix the problem or they will be able to ensure that you are going to be sent a new sim. With a pay as you go phone, you are going to have to buy a new sim altogether, you cannot simply request a new one. Before you do any of this it is going to be a good idea for you to ensure that the problem is with your sim and not the phone itself. You are not going to have to write any letter to request a new sim; you are simply going to have to follow the instructions that are in place to ensure that you can have the problem fixed as soon as possible.
Checking the sim first is always going to be a good way for you to ensure that the problem is not with the phone and therefore you are not going to have to have a new phone as well. Handsets and sims are completely different technology and you need to know which one is making the mistake. For both problems it is going to be within your benefit to go into a local store which is under your service provider and to see if they can help you in any way. Failing this you are either going to have to buy a new sim or speak on the phone to a member of staff who can order you a new one and have it delivered to you.
Checking the sim first is always going to be a good way for you to ensure that the problem is not with the phone and therefore you are not going to have to have a new phone as well. Handsets and sims are completely different technology and you need to know which one is making the mistake. For both problems it is going to be within your benefit to go into a local store which is under your service provider and to see if they can help you in any way. Failing this you are either going to have to buy a new sim or speak on the phone to a member of staff who can order you a new one and have it delivered to you.