How do you use FaceTime on an old iphone?

To use FaceTime on an old iphone, ensure that the device is running at least iOS 4 or later, as this is when FaceTime was introduced. Open the FaceTime app, sign in with your Apple ID if prompted, and then enter the email address or phone number of the person you want to call. Tap the video or audio icon to initiate the call. Ensure you have a stable internet connection via Wi-Fi or cellular data to successfully connect.