回头 here means 'later / in a while', not literally 'turn the head'. It's vague on purpose — anywhere from 10 minutes to a few hours. 再打 = 'call again'. The whole phrase is the polite way to end a call you can't take right now.
memory
回头 (literally 'turn-head') took on the time sense from the gesture — turning your head back at something later. Same metaphor in 回头见 ('see you later').
