Air Canada makes it a bit difficult to find their phone number on their website and probably re-arranges their website too frequently which confuses the Google computer scripts.

So, here are the important Air Canada numbers, all on one page:
In Canada and the US, call 1-888-247-2262

How to Navigate the Air Canada Telephone Prompts

Press "1" right away for English
Then
"1" for checking departure and arrival times
"2" to search for flight fares
"3" for Frequently Asked Question
"4" to book a flight or make changes to your reservation
"5" for help with their website
"6" for flight passes

Air Canada has a list of their overseas (local access) contact numbers here:     http://www.aircanada.com/en/customercare/int/. Be sure to keep a copy of the ones that you'll need. If you want to keep things simple, this is the number that should work from anywhere, 1-514-393-3333, but it will also be long-distance from almost anywhere.    

If you want to complain to Air Canada, they'd rather you email them than call them, and here's the place to do it (although it's technically not an email). Be sure to keep a copy of any and all correspondence with them, I suggest sending yourself an email so you can't lose it.  
   
If you're travelling, don't forget to bring a copy of the Visa emergency card services numbers, for Mastercards, you'll have to just look at your card.