Yes. To keep your number with a new SIM you will need to call your current mobile operator and ask for a Porting Authorisation Code (PAC).
The PAC is a 9-digit number and you should receive it within two working days.
Use the PAC when buying a SIM Only contract, or afterwards by calling your new provider, to keep your old number.
Your PAC will expire 30 days after obtaining it, so don’t wait too long.
You can’t use the number of a disconnected phone, so make sure to change numbers before you cancel your old contract.