The phone number change should definitely work, as long as you have account holder access to Fido.ca.
Have you tried a different browser to see if it works?
Let us know, and if it does not work we can send you a PM so we can take a closer look.
I tried with chrome, internet explorer, microsoft edge and safari. It's not working for me. I am choosing a same area code. I have a valid account with good standing.