Starting July 29,2012 dialing only 7 digit phone number for calls within Manitoba will cause one to hear an announcement that one should use 10 digit dialing. Your call will still go through.
Then on October 20,2012 10 digit dialing becomes mandatory. If you do not dial 10 digits for calls within Manitoba your call will not go through and instead you will hear a reminder to use the area code.
for more information regarding 10 digit dialing please see here