User busy call disconnect

You have Do Not Disturb turned in. You can turn it off in Control Center with the button circled in green:

You can also customize Do Not Disturb. Do these steps ONCE:

• Tap Settings
• Tap Focus
• Tap on any Focus settings that you will never use and remove them (you can add them back later if you change your mind, and even create new, custom ones)
• Tap Do Not Disturb
• Tap People
• Tap Allow Notifications From
• If anyone is listed who should not call you when DND is on delete them; if you don’t want any calls from anyone, any time, delete all people
• Tap the dropdown next to Allow Calls From and choose Allowed People Only
• Optionally, tap on Apps, then Allow Notifications From and don’t add any apps.

All of the above is a one-time setup. After that, turn DND on or off with the Control Center Focus button. You can optionally set one or more schedules by going to:

Settings/Focus/Do Not Disturb/Set a Schedule.

Well, that is a feature of DND; if that isn’t the problem it is a problem with your carrier, so call them.