Download WhatsApp before you get to Costa Rica. It is widely used to communicate down there and is helpful in some cases- that’s how my classmates and I communicated while we were there. It works on Wi-Fi, so you don’t need a phone plan.
Not all homestays have Wi-Fi, but the EcoCampus and La Casita Azul do, so you’ll have access to it every day once you get to the campus or the library.
Phone plans: NOT necessary, but some people get them. I got one because I was travelling down to campus a day later than everyone, so I needed to keep in contact with my professor and the campus. Wi-Fi can also be finicky, so if you’re someone who needs to keep in contact with your parents or kids at home, I’d suggest getting a cheap plan just in case- like if you don’t have access to Wi-Fi on day or overnight trips.
If you need to keep in contact with people at home for reasons like you have kids, I’d suggest telling the Las Nubes semester abroad coordinators, like Ana Martinez. They’re really good with making sure you’re set up in a homestay that has Wi-Fi.