1. I promised to call back within an hour.
2. I promised to call back in an hour.
The two sentences have different meanings.
Let's say you made the promise at 10.00.
'Call back within an hour' means you will call back anytime between 10.00 and 11.00.
'Call back in an hour' means you will call back at 11.00.
not a teacher
Student or Learner