If a phone rings once and then goes to voicemail or rings only briefly, it You can't definitively know the cause of a phone barely ringing and going to voicemail because neither a phone company nor the phone's operating system provider will tell you if your phone number is blocked.
How do I listen to my own voicemail greeting on iPhone?How do I turn off voicemail? Looking in the voicemail settings and searching for a ‘Deactivate‘ or ‘Turn Off‘ option is an obvious first step regardless of the phone you’re using. If you find the correct option, disabling it will turn off the voicemail function.

I called customer support. They were not able to fix it and made it worse. Now it rings once instead of three times. They put in a service request and said it would be 1-3 days. Now they say 3 to 5 business days. I don't understand how something so simple could be such a major problem. I just wanted them to change the number of rings to at least 7. Disconnected the 1st of the junction boxes at top of door. Phone line rings after doing this so proves problem is in the home. I then ran a new wire pair to the NTE5, problem then came back, even with the pull out isolation port or whatever it’s called. Then wired direct to the back plate if that’s the correct term, phone starts ringing again.
Go to the Settings app, switch Airplane Mode to off, then switch it back on 30 seconds later. If that doesn’t work, go to Settings > General > Reset and select Reset Network Settings. You also might want to try powering down your iPhone and turning it back on. Select the Voicemail tab, then select Set Up Now. Note: Alternatively, you can set up voicemail by selecting the Phone app then pressing and holding the 1 key. If prompted, enter your voicemail password then follow the prompts to set up your voicemail. Go to Settings app → Tap on Cellular Data → make Turn Cellular data ON. If it’s already enabled, then make the turn-off and back on again and however, you have an issue regarding voicemail then try next way.