Fido is having access to Telus/Bell network as (Fido-EXT) and can use their network to make call/text as normal. However, it can only switch when only your phone has no Fido service for a while. Also, I think there are some restrictions on data speed/call quality with FIDO-EXT. However, If you are in an area where Fido service was so bad that you would like to use other network (the Fido signals was too weak and Telus/bell signal was overwhelmed) you might want to force phone to use the Fido-EXT.
In iOS go to Settings >> Carrier >> Disable Automatic >> Select Fido-EXT.
In android Settings >> More networks >> Mobile network >> Select Fido-EXT.
Again, this only apply when you are in a bad spot for Fido network, and other carrier signals are good.
Switch back to Fido or automatic for the best experiences.