Hanging up by itself

I’m going to post the full list of things to check (some have already been discussed):

• The Mute switch on the left side of the phone. It should be towards the screen (no red showing if you look at it)
• If you have a case on the phone verify that it isn’t interfering with the switch
• Do not Disturb - If there is a crescent moon in the top bar in Control Center go to Settings/Do Not Disturb and turn it off
• Check Do not Disturb - Scheduled
• Check Do not Disturb - DND while driving
• Volume - with the Home screen displayed press the Volume Up button (on the left, just below the Mute switch) and watch the volume setting on the screen.
• Settings/Phone - Silence Unknown Callers - if this is on calls from numbers not in your contacts will go to voicemail
• Settings/Sounds & Haptics - Make sure there is a sound assigned to Ringtone
• Is it calls from specific individuals? Make sure that they have ringtones assigned in Contacts
• Do the missed calls go to voicemail?

Your iPhone will decline calls if you have Do Not Disturb enabled. When it is enabled you will see a crescent moon in your status bar in the Control Center. You can control it by going to Settings/Do Not Disturb. There are two switches, one at the top that is unconditional, and one below for Scheduled. If both are off none of the other settings matter.

Another possibility is in Settings/Phone - Silence Unknown Callers. If this is on then calls from numbers not in your contacts will be declined and sent to voicemail.

You should also check your Recent Calls list by tapping Phone/Recents. If the declined calls do not appear in the list then your carrier is not sending them to your phone. If this is happening you must contact the carrier to resolve it.

Did you check EVERYTHING I posted, including looking at Recent Calls? Are the calls that are declined shown in Recent Calls?