I had been puzzled for ages why my iPhone was not allowing calls and sending them to voicemail immediately. I had checked all the usual settings, My voicemail settings, the Phone settings. I could not see what the issue was.
Then I remembered that due to an important meeting I was in I had activated the “Do Not Disturb” function on the Control Panel:-
Now this caused the problem. So beware accidentally switching this feature on because any incoming calls will go straight to voicemail. Due to this I am not sure its such a great feature to have so visible on the phone. Why not keep it in Settings and not on the control panel.
For more information about this feature go to:-
https://iphone-tricks.com/tutorial/205-how-to-set-up-do-not-disturb-on-iphone
AccessAbility Solutions 