Hello Marvellous1,
Welcome to the community!
To clarify, are you new to Fido and got a phone number provided from them? I understand it might be alerting to find out the number you received may have already been registered for other services, such as WhatsApp. However, that can be a normal situation.
The number of phone numbers is finite. From time to time, new overlays are created to expand the existing number allocation. However, with the number of users ever increasing, even those overlays fill are quickly exhausted. The practice of recycling numbers is commonplace and necessary. Generally, there is a minimum period where those cancelled phone numbers are not in circulation (see here). So while the number you received from Fido may have been previously been owned by someone else, it wouldn't have belonged to anyone else when it was given to you.
With regards to the number being previously registered to WhatsApp, the previous owner of the phone number did not deregister that number from WhatsApp (see here). If you are unable to register that number with WhatsApp, you might consider contacting their customer support and inform them that you are the new owner of that number.
With regards to SIM swapping, it generally involves fraudulently porting a number from a different provider to a new SIM card. In order to prevent SIM swap fraud, the mobile providers have implemented a system whereby a verification text response is required or else the transfer will not proceed (see here). You can view additional information regarding protecting yourself against this type of fraud here and here.
Hope this helps 😀
Cheers
