Call, email, text or Facebook - all are acceptable in today's wired world. If someone takes offense to your method of communication, they will let you know. If the broker has a showing service, that is the method I use, I get automatic confirmations, showing details, and I can easily click on a link and provide meaningful feedback. Recently, I completed a deal with another agent who does not use email ... I must say, it was one of the least efficient uses of my time, faxing and meeting him at the office with hard copies.