It seems like no one can reach you when calling your iPhone. The reason for this is that your iPhone is in Do Not Disturb mode, and I’m not talking about the hardware switch. I’m referring to the Do Not Disturb mode that you activate in the settings. The issue is, unlike older phones (think landlines and basic cell phones), or even most smartphones running Android, when those devices are in Do Not Disturb mode, the caller still hears the regular ringing tones. Not on an iPhone, though. When you put your iPhone in Do Not Disturb mode, it will immediately hang up all incoming calls.
This means that the caller will hear a busy signal, as if you’re already talking to someone else. If you have voicemail service set up with your carrier, the call will be immediately redirected to voicemail—something that’s different from how a regular phone works, which would typically give a couple of rings before the call is sent to voicemail.
So, when someone calls your iPhone while it’s in Do Not Disturb mode, the device will show a notification for either a missed call or a new voicemail, depending on your carrier, service, and settings. You can also choose to disable these notifications or allow calls from repeated callers.
In short, if no one can reach you on your iPhone, there is nothing wrong with your phone, carrier, or reception. All you need to do is deactivate Do Not Disturb mode.