There is an emergency cell phone located at the house with good 3G signal. The one you bring from your own country will likely charge you high international rates. With an older 3G phone, you can go to the local ICE card and they will swap in a Costa Rican SIM card, and you can get Colby pre-pay minutes for local calls. Texting with this is much less than making voice calls, only a few colones/message.
The cheapest way to use your home country smart phone is to buy an international pre-pay plan or one with texting. Until recently , everyone rented cell phones when in Costa Rica because they hadproprieary SIM cards. This is changing.
To call back to the US, I use Skype for free, or a Magic Jack, also free when you have a wifi connection.
Ask your rental car company to provide you with a cell phone for local calls. If you rent a phone in San Jose, get the GPS ones, as they work best in southern Costa Rica.