When entering a phone number into the notes field, the number automatically becomes a link to the phone application.
Just add a phone number to the notes field of a call task and the call can be made directly from the application. When the call is finished, OmniFocus boots back up.
This also works for synced tasks. In other words a phone number entered in a note field of the computer version of OmniFocus, will automatically convert to a phone link upon syncing.
This can be useful, for example, when you plan on making a series of phone calls away from the computer.