When your phone only rings once then cuts out, this fault is known as "ring trip". It can be caused by a number of things either inside or outside your property, and will require a process of elimination to track down the cause of the fault.
Open Settings → General → About → If there is an update, you will see a pop-up. → Tap Update.

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On the home screen, expand the App Drawer and tap on the entry named Visual Voice Mail. You’ll be taken through some introductions screens that will showcase some of the capabilities of Visual Voicemail. Tap on Start to configure the service.If you’re a new customer or you haven’t gotten around initializing your voicemail, you will be prompted to do so. But if you followed the first guide, that won’t happen. In any case, after you enter your voicemail password, tap on End Call. After a short waiting period, your Visual Voicemail should now be configured. From the next screen, you can Set your personal greeting or hit Ok to skip this step. To check a voicemail with this service, open the App drawer from your home screen and tap on Visual Voicemail.You’ll see a list with all your voicemails. You can select them All or tap the New tab to only display the ones that you haven’t listen to so far. After you open a voicemail, you can listen to it by tapping the Play icon. To delete a message, tap the trash icon then tap Delete.