Practically all professionals in the service industries have email. In most cases when phoning a Mexican businessperson, you will have to go through 1-3 secretaries before reaching the desired person. These secretaries may not speak English, so have a short sentence in Spanish prepared, asking for the specific person. In Mexico, time is not money. They may take their time to reach a decision.
Bureaucracy is big in Mexico.
Always take with you copies of most relevant files and legal documents that may be required during the transaction. It is also very important to take photocopies and originals of your passport as proof of identification when you're closing a deal. It is your duty to keep abreast of the latest legislation. It is possible to buy specialized selections from it according to line of business. It can be purchased and subscribed to on-line.