Every SIM card has a unique serial number, the SN number. At first sight, the SN number looks like an ordinary phone number. If your number was transferred from another provider in the past, your smartphone will often only display this SN number. What actually happened, is that this serial number was replaced by the phone number you transferred from your previous provider. The SN number is not an active phone number, when you make calls or send texts your contacts will only see your 'real' telephone number. So, nothing to worry about.