Assuming you want a Canadian Number. Not sure if it is still current but the one I have is 1-800-782-3387. They take a listing but the load info they give is not very helpful. I've been told "only 1 over, you should get on no problem" then all show up and I haven't got on.
If it's YYZ-LHR you want I would suggest Air Canada, they have more flights a day(5 this time of year) and ZED tickets are cheaper. Usually no problem getting on. Call 1-888-247-2262.
Hope it works out!