There is a red dot on my iPhone's screen, why?

Check that you have not (voluntarily or involuntarily) activated Focus - Do not disturb.

Should be this case, turn it off, as per these instructions:--> Turn off Do Not Disturb - Apple Support

Regards

Giulio

1. Go to Settings > Notifications> Phone> then turn OFF Badge App button. The red dot should disappear from the phone icon.
2. Check if the voicemail is working (which it should be). Call yourself from another phone and leave a message > Call voicemail and delete your message.
3. Go back to Settings > Notifications> Phone> then turn ON Badge App button. The voicemail should now be working and the phone icon should now start showing the badges when you received an email.

You're most welcome :)

Maybe you had an incoming phone call that you didn't answer, or a VoiceMail you didn't read.

Open the phone app and tap on the Recent icon. If there is a lost call, delete it (swiping from right to left on it), then restart your iPhone.

Regards

Giulio