Yes, you can retain your old phone number if you are taking out a sim only contract (even if you are changing to an entirely new network). To keep your old number you would need to contact your old mobile phone network and ask them for the PAC (Port Authorisation Code) number. Once you have this, you need to contact your new provider and give them the PAC code within 30 days. Your mobile phone number will then be transferred onto the new network and you can use it via your sim only contract.