If you are a customer of Aliant, MTS, Bell, SaskTel, or Telus, this is the cheapest way to phone home:  go to any TELMEX public phone (usually blue and white ones) and dial either 01 800 123 0200  or  01 800 021 1994.  This will give you a direct connection to  Canada Direct... then. you can call collect or use your calling card.  If they request credit card, you are not connected to Canada Direct.

An even better solution (cost less and is more convenient) is to rent a cell phone from Mexitel Cellular. Calls to Canada and the US cost only a flat $1 per call, incoming calls are free, and they send the phone to you at home (or office) before you leave. (In some places like Cancun you can pick the phone up there also.)