Many cell phone carriers in the United States do not provide service in Cuba, and if they do, roaming comes at a very steep cost. For instance, I have Verizon, and my plan includes international data in nearly every other country for $10 a day. However, Cuba is not included with this; everything would be done by the minute, by each text, and by each MB if I turned on my data.
So basically: calls for me were $2.99/min and it was $0.50 to send a text, $0.05 to receive a text, and $2.50 per MB.
To put this into perspective, just sending an email with a couple of pictures can be 500kb (half a MB), according to POPSUGAR IT. "You also have to take into account that apps are constantly syncing and updating in the background so just turning your phone off of aeroplane mode can be well over 50 to 100MB," said Desktop Support Technician Rodrigo Urbina.
It was still nice to have this option for backup in case of emergency, but it's much more economical to just connect to WiFi when you can and make calls that way using services like Facetime voice or video, Skype, and WhatsApp.
Another Option: You can buy a burner phone when you get to Cuba, which will allow you to make calls solely within the country (which can be good for emergencies and logistical planning purposes!), or you can look for casa particulares and/or hotels and hostels that offer the usage of a phone there. Some even offer a cell phone to be used for the duration of your stay!
Image Source: POPSUGAR Photography / Hilary White