Welcome to iMore! I'm assuming you're referring to a missed call badge on your Phone app that's getting stuck. Usually viewing your recent calls and voicemails will clear it up. On a very rare occasion mine notification badge gets stuck but it usually disappears soon after. Have you tried conducting a hard reset? Reset your device by holding the power button and home button down (or volume down button on iPhone 7/7+) simultaneously until you see the Apple boot logo. If it keeps happening then contact Apple Support or visit an Apple Store.