From your hotel, it is best to call using a credit card. If you charge your call to your hotel room, check on surcharges before, which are sometimes over the top.
Almost all hotels have Internet connectivity, usually for a surcharge of approximately $15 per day. For those visiting UFM, the campus is wireless, so if you have a laptop you can hop on.
Voltage throughout Guatemala is 110; outlets are the same as in the United States.