Blue pay phones (2 kinds) - work with phone card ($10 & 20) available at post offices and many tourist stores;
the others are coin phones but rarer. You must use the blue phones for international calls (expensive); try to use the Cuban phones for calls within Cuba.
Silver pay phones (2 kinds coin and card) are for Cubans and much cheaper. They require Cuban coins – the 20-centavo (1-cent) gets a couple of minutes local call; that’s the reason to have a few peso coins. The REAL useful thing to have is a Cuban 7 peso phone card for that version. A long distance call within Cuba costs a few pesos on that card vs about the same in dollars using a blue phone.
BlueTelephone Booths on Street usually have a REALLY bored, pretty but stand-offish attendant who should be able to help make a call.
Newspaper Kiosks with phones.
Telephones in Hotel lobbies are useful if you are having trouble making a call; they might dial it/speak for you. They charge a premium but it can be worth it; I had a desk clerk book a taxi for an early morning flight even though I was staying in a casa down the street but wanted to be sure it would come.
wild bill