If you have a US cell phone, check with your provider as some offer an International calling plans..especially to Mexico. We have AT&T and pay the same year round for calls to Mexico from the US, or from US to Mexico, regardless, for the same rate as we would pay in the US. I believe it is called the VIVA Mexico plan. You can change your plan at any time once you get back to the US.
Either that or get a calling card, but your cell phone provider would be easier.