Man, Fido did some similar BS with me a few years ago! As long as I called and complained about the error on the bill I'd get amended bills or refunds if I had already paid.
One time Fido actually owed me money, $250. They were due to pay me back after six months of service (it was a security fee). It took them 13 months to pay me back. I bitched them out of course and then argued that if I had owed them $250 I would have been swamped in late fees by now. They offered to waive my cell phone fees for the next two months and gave me $60 worth of talk time, so I took the deal. I kept my eyes on every bill ever since.
My last trip to Canada I went with a prepaid/pay-as-you-go phone.. It was F'ing expensive, man. It cost me $1.50 a day just to receive calls and texts! That should calculate to $45 a month. If I made any calls that meant needing to recharge my credit sooner, so basically I charged my phone with $50 roughly twice a month.. Now we're talking $100 a month for the absolute bare minimum phone service just because I went with prepaid in Canada. I would have gone with a plan but I had no choice since for some retarded reason Fido told me I needed either a Canadian passport or a Canadian residency card.