When using your mobile phone, you become local to whatever cellular site processes your call, regardless of your mobile phone number or what your local calling area might normally be. If long distance applies, you're charged from the cell site location that processes your call to the location you're calling.
For example, if your mobile number is 416-570-XXXX (local calling area normally being Toronto) and you're in Sherbrooke calling a Sherbrooke number, the call is considered to be local and and you’ll be billed for usage but no long distance charges will apply.
If you're in Sherbrooke calling Toronto, you'll be billed usage plus the long distance charges from Sherbrooke to Toronto, even though you have a Toronto-based mobile phone number.