Calling a restaurant on the phone to make reservations is downright barbaric. The background noise is off the charts, the hostess has better things to do than talk to you (and it shows in her tone), and trying to nail down your preferred time can be an exercise in futility.
OpenTable helps you skip all the nonsense and get right to the point what's nearby, which times are available, and how are the reviews? Once you find a restaurant that looks good and has an available table, tap to reserve it. Done and done.
See also: Urbanspoon, Foodspotting, Restaurant Finder