Getting a mobile phone in Guatemala is easy, you go to a mall, or a shop everywhere, buy one, and it will be working the moment you get it, provided it is charged. There are three phone companies Claro (part of the Americatel Mexican group led by Carlos Slim), Movistar (Telefónica Spanish Group) and Tigo (Millicom Group for Luxemburg), and all three offer reasonable service, if you are going to a remote place for a long time, it will be good finding out which company has the best coverage there, as it varies from company to company.
Prepaid phones cost from $25 to whatever you wish to pay. Usually they include minutes, that void after a period of days. You can buy calling cards in the street, supermarket, and almost everywhere else. They cost from less than US$1, up to around $25. Minutes are void after a period that depends on the price of the card. Every two or three days, they will give three times of what you pay for, but the bonus will only last for a very limited time. Local and US calls are around 10 US cents a minute, and a little more expensive elsewhere. If you would like to buy a contract phone, you will get a decent free phone, depending on the service you would like, but it will be subject to a credit approval.
If you are in luck, iphones or android phones will be in stock, but they will cost you more. All companies use sims cards, and depending of the phone you buy and the price you pay, can be used in any phone or locked to a phone company, sometimes sims are pre-soldered so you cannot remove it, but only in very cheap phones.
Driving and talking in the phone is prohibited, and Transit Police will stop you and fine you if they see you. Burglars will also stop you, but they will take the phone from you too (not kidding), so you will better not talking in the phone while driving.
4G/3G/Edge Internet is also available, and you can easily buy the USB modems, or use the Internet in your phone, but be careful if you do not activate a data plan, and try to use the Internet on your phone, they will let you, and charge you zillions.
There is no phone signal at Tikal national park, except at the top of some of the big pyramids, because placing an antenna inside the park is forbidden.
Unused phone lines usually expire after three months of no use, even if there is money left in your account.