Toll free numbers don’t normally work from outside the country as the receiver is paying the toll charges and most companies or individuals are not willing to pay international long distance rates for such calls and there are likely not the requisite conventions between telecom carriers in different countries for payment to be made for the call.

US 1800 numbers don’t work if you try calling them from Mexico either.

You will likley have to somehow patch through a regular number in Mexico which can then dial the 800 Mexican number. Alternatively you will have to find a regular toll number for the company or individual.