Most people prefer to write emails explaining their decision, which usually works and is enough. But if you want to go beyond expectations, you can call them. Doing so avoids miscommunications that might emerge from emails, and it gives a personal touch. One thing to avoid is sending a text message, as it’s too informal and might be seen as unprofessional.
Check out below an episode from a podcast about career advice in which a recruiter talks about the best way to deliver the message (for specific information, go to 7:35)