There is no longer a Fido network. Your phone is constantly roaming on
Rogers's network. And the phone has absolutely no idea if it is roaming in an
area formerly covered by Fido or not since the towers emit the same network ID
(CAN 072)

Only Rogers knows which towers will result in the "extended coverage" and
which won't. There is now a manual too for this, you need to dial *#123# and
it then tells you if you are now in a fido basic coverage area or extended
coverage area.

I am not sure if the 4 legged furry CSRs would have an option on their
copnputer to disable roaming of your account into extended areas. You would
have to call them. The other option is to turn your phone off or set
forwarding to voicemail for all calls.