SIM swap is a genuine service which allows you to keep your existing phone number and change between different SIM sizes or phone providers.
It is also a technique which is becoming increasingly common amongst fraudsters as it can provide them with the ability to utilise your mobile phone number, benefiting from all the functionality and services that would provide, such as receiving and making phone calls, receiving and sending SMS messages as well as using any provisioned data allowance.
If you have any concerns you should phone your telephone provider immediately to confirm whether a SIM swap has been undertaken, if it is confirmed you must phone your Bank(s), who may hold your mobile number as a contact for you.