Contact:
You can call from public phones using prepaid tel. cards tarjetas ladatel, bought at magazine stalls. Cards can be purchased in $30, $50 or $100 pesos denominations. The rate to call the US is roughly equivalent to $USD 0.50 per minute. Beware these are different than tarjetas amigo, viva, or unefon: they are for cellphones.
Some areas have only a few internet cafes; in others, they are plentiful. Common fees vary from 7 pesos/hour to 20 pesos/hour. Currently, most of the internet cafes offer calls to the US for a better rate than a payphone, usually via VoIP.
If you have an unlocked GSM phone, you can buy a prepaid SIM card in Mexico and have a local mobile phone number for use in cases of emergency. Telcel provides good coverage throughout the country and you can get a SIM card for $150 pesos with $100 pesos talk-time. It's often far cheaper than what hotels will charge you and incoming calls may also be free under certain schemes. Mexico operates on the same GSM frequency as the United States, 1900 Mhz. theres an internet wireless conection in almost every restaurant or hotel in the big cities.

Stay healthy:
Mexico is so notorious for traveler's diarrhea that it is often called "Montezuma's Revenge" (Venganza de Moctezuma). The reason for this is not so much the spicy food but the contamination of the water supply in some of the poorer zones in Mexico. In most of the small towns that are less industrialized, only the poorest Mexicans will drink tap water. The best policy is to only drink bottled or purified water, both of which are readily available. Just like in the USA, in most major Mexican cities the water is purified at the cities' water company. In most restaurants in these poor zones, the only water served comes from large jugs of purified water. If you get sick visit your local clinic as soon as possible. There is medicine available that will counter the bacteria.
Tap water is safe to drink in Mexico City, Monterrey, and several other major urban centers. Most travelers will tell you that it is not even though they have never even tried drinking it! If in doubt, just avoid drinking it or take mind of how you feel afterwards.
Medicine in urban areas is highly developed, public hospitals are just as good as public hospitals in USA, and just as the American public hospitals, they are always full. It's recommended going to private hospitals for faster service.
In remote areas consider carrying a first aid kit, aspirin, and other related items are sold without medical prescription.