To keep your current number when you switch to a Sim-Only deal you will need a PAC code from your current supplier.
Getting hold of your PAC code is now much easier than it was in the past. All you have to do is text ‘PAC’ to 65075 and your network provider is required to send your PAC code in a text reply as soon as possible. In some cases, it may even be sent right away.
You can then send the PAC code to your new provider, and they will arrange to carry over your current number.