When your iPhone is locked, incoming calls display a 'slide to answer' control instead of a decline option. Once unlocked, Accept and Decline buttons appear—either in a compact banner or across the full screen, depending on your Incoming Calls setting. This is default behavior and not a glitch.
You can still decline calls quickly by pressing the side button twice to send the caller to voicemail, or press a volume button to silence the ring without declining. For a more polite rejection, use the Message option to send a text reply instead. You can customize prepared responses in Settings > Apps > Phone > Respond with Text, such as 'I'm working right now' or 'Please text me; I'm busy.' To block unwanted calls, go to Recents, tap the number, and select Block Caller.
In iOS 26 and later, enable Screen Unknown Callers to filter spam, with options to never screen, ask for a reason, or silence unknown calls entirely.
Source: Engadget · Summarized by HeadlinesBriefing