This situation may occur on various iPhones, and it could be because you accidentally activated the Do Not Disturb mode or activated it but forgot to turn it off. The Do Not Disturb feature was developed from iOS 6 and is very useful for users who don't want to be disturbed by phone calls while busy with something. Also, check out the article on fixing iPhone 6 not detecting SIM for more information.
Firstly, observe the area near the battery percentage display on your iPhone 6. Is there a crescent moon icon below? If yes, you've definitely turned on the Do Not Disturb mode. Follow the steps below to resolve this issue.
Step 1: Access the Settings section.
Step 2: Next, select Do Not Disturb.
Step 3: In the Manual section here, swipe left to turn it off.
After performing the above steps, try taking another phone and call your device to see if it receives calls normally.
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So here, we have guided you on how to fix the issue of not receiving calls on iPhone 6, 6 Plus. The method mentioned is not actually an iPhone issue but a feature you accidentally activated, preventing notifications. If you own an iPhone 5s, you can apply the same method. However, if you have an iPhone 5s running a different iOS version, you should perform similar steps.
