The competitor will make the arrangements to port out the number from Shaw, and the service will be cancelled.
@oneoldman -- you might get contacted by your current provider, for them to "release" your phone-number to the competitor.
Also, if you currently are a Shaw Home Phone customer, remember to return the Shaw Home Phone adapter hardware, to avoid being charged for it, by Shaw.
Consider transferring your phone-number to Shaw's new (as of July 30, 2020) "Shaw Mobility" service. If you don't want to use "cellular-data", and you have a compatible, unlocked, mobile-phone, the monthly price will be ZERO ($15/month, but with a $15/month discount, if you sign-up for automatic payment against your credit-card).