Another excellent article in Phone+ about Tariff pricing superseding contract pricing. That's right, if you have a contract with Bell but the tariff rate is higher you can be charged the tariff rate, despite the signed and executed contract.
"Thus, for example, if a carrier offered a reseller a rate in a contract that was not found in its tariffs, that carrier could not lawfully provide service at the contract rate. Indeed, the Supreme Court has repeatedly held that even where a carrier intentionally (fraudulently) misrepresents its rates in a contract and the reseller specifically relies on the misrepresented facts, the carrier can not be held to the promised rate if it conflicts with the published tariff, regardless of the negative impact on the reseller."
Luckily (sarcasm), most services are not in the tariff, so the contract pricing is usually valid. And in the case of a CSA, it was signed off by the PSC (in theory) and became a de facto tariff. In the case of an non-tariffed service that is in a service guide (like Metro E), contracts that refer to the service guide leave the door open for the service guide price to be the billing rate. Please read the article by Neil Ende of Technology Law Group, LLC.