To remove Mr. Number from your iPhone:
1. Exit the app and long-press the app icon.
2. An "x" will appear in one corner. Tap the "x" to completely uninstall.
If you are still having issues after uninstalling Mr. Number you might have to reinstall the app and then try these steps:
1. Disable Mr. Number in the iPhone Settings (Settings->Phone->Call Blocking & Identification)
2. Force-quit Settings
3. Uninstall Mr. Number
4. Restart your device