Redial is a one-touch function to re-call the last dialed number. When the line is busy or unanswered, it re-dials without re-entering the number. Landlines have a "Redial" button; smartphones use call history to tap the last number.
Redial dates back to push-button phone era. Rotary phones lacked this feature, requiring full re-dialing each time. Electronic phones first enabled storing the last number in memory for one-touch re-calling.
Auto-redial automatically retries at intervals when the line is busy. Useful for ticket reservations and popular restaurant bookings, but excessive auto-redialing strains the recipient's line and should be used considerately.
Smartphones integrate redial into "call history," enabling re-calling any past number. When receiving suspicious calls, don't redial blindly - search the number first to verify the source as a special fraud precaution.