If your iPhone rings but doesn't vibrate when someone tries to call you, it's possible that the Vibrate on Ring option is disabled. Follow the below-mentioned steps in order to enable this option:
Go to Settings and select Sounds. Move the slider next to Vibrate on Ring to the right.
It is very important to update the device's operating system on time as the updates contain fixes to various bugs. If you don't receive a notification about the update, check if one is available manually.
See the don't know how to update my operating system guide if you are not sure how to check for updates manually.
Maybe the vibrator got stuck. A gentle tap on the back of the device should be enough to unstuck it.
If your iPhone doesn't vibrate nor ring, but you receive notifications in your voicemail when somebody calls you, it's possible that you have enabled the Do Not Disturb mode. If you see a half-moon icon next to the battery icon (in the upper-right part of the screen), this option is enabled.
You can disable this mode either via the Action Center or via Settings:
If you don’t receive the email in the next 2 minutes please check your junk folder, and add [email protected] to your safe sender list.