All local land lines in Utila start with 2425 and then have 4 numbers after. Like 2425-3993. To dial locally, you just dial the number in full. To dial internationally, you need to add the country code (504) and then any access code from where you are dialing. From the States its (011)-504-2425-3993.
Most local cell phones start with a 9 or a 3. That is how you can tell they differ from a local land line. If calling from out of country, you need to add the country code 504 before dialing the cellular number.
Be aware that most folks on island don’t check their cell phone messages and many won’t even know their numbers which is pretty normal around here!