To port your number to another network, whether contract with phone, PAYG or sim only, you will need to get a PAC which is valid for thirty days from when it is issued. When you try and get one from your existing provider, they will try and do everything that they can to stop you getting it and keep you as their customer. They will ask what you are looking at doing, so it's an idea to know what's out there before ringing. The department that you get the PAC from will be able to offer the best deals that your network has to offer, because they are last chance saloon to keep you as their customer. If you can say that you are moving to "x" because they are offering you a, b and c for x pounds a month, then they should be able to at least match it, and may beat it. They will be able to offer you sim only deals or an upgrade phone on a good deal. Good luck