Forget where the number of your mobile is based, it has nothing to do
with the calculation of long distance.  LD is determined by the
relationship of the tower you are accessing to the exchange you are calling.

Toronto cell answered in Montréal = LD
Toronto cell answered in Toronto = Local
Toronto cell Montréal call to Montréal = Local
Toronto cell Montréal call to Toronto = LD

For INCOMING CALLS:

The call is routed to your home city first because your phone number
dictates how calls are routed on the landline systems. When Fido is
handed the call, Fido looks up its database for the tower currently
handling your handset and routes the call to that tower (or in the case
of roaming, to the network handling your handset).

Fido then decides whether a call from your home city to your current
location should be billed a long distance rate, or the roaming rate to
the country you are in.

Within metro areas, the cellphone networks often totally bypass landline
networks so they can have different local calling areas. (when Fido
started in Montreal, it had one big switch in Montreal, so all fido
towers from St-Jerome to the South-Shore were considered to be local
calls, whereas Bell would charge long distance for somone calling across
the river from Pierrefonds to Laval.) However, once Fido started to
decentralise its switches, it was able to break up the local calling
areas to better match Bell's calling areas. But it is still all very
subjective decisions because within urban areas, the vellphone networks
have their own networks of leased lines and microvave repeaters.

For OUTGOING CALLS:

When you make a call, Fido decides what city you are in based on the
location of the tower serving you. And it has tables of where you can
make local calls from that city. If you call outside that area, they
ding you for long distance.

When you are roaming, say in New Jerseay on T-Mobile and make a call, it
si T-Mobile that decides if your call is considered long distance or
not. It sends billing information to Fido for long distance and airtime
for that call. Fido then adds its now huge markup and bills you.