If you've noticed that your iPhone has two lines whenever you're on a call, it's likely because you have enabled the "FaceTime Audio" feature. This feature allows you to make audio-only calls using the FaceTime app on your iPhone. The two lines that you see on your iPhone during a FaceTime Audio call represent the audio channels of the call. One line represents the audio coming from your iPhone's speaker, while the other line represents the audio coming from the earpiece.
If you wish to disable this feature and go back to the default phone call interface, simply open the Settings app on your iPhone, scroll down and tap on "FaceTime," then toggle off the "FaceTime Audio" option.