Half ring then to goes to voicemail.

FIRST: Are the missed calls in Phone/Recents on the phone? If they are not the problem is in the network, because the calls aren’t getting to your phone. Contact your carrier: 

• check with your carrier that they don’t have a local outage, and that you didn’t have any settings that would have immediately invoked voicemail
• check up on them at Down Detector - outages

If they are in Recents here’s what to check:

• Check the MUTE switch on the left side of the phone, above the volume buttons. It should be towards the screen
• Check the volume setting. With the Home screen displayed, press the VOLUME UP button on the left side of the phone. It’s above the VOLUME DOWN button.
• Go to Settings/Do not Disturb. Turn it off if it is on. Also turn off Scheduled. With both off none of the other switches there matter.
• Go to Settings/Phone. There’s a switch for Silence Unknown Callers. If it is on any calls from numbers not in your contacts will go directly to voicemail.
• Also in Settings/Phone: Make sure Call Forwarding is off.
• Go to Settings/Sounds & Haptics. Make sure there is a sound assigned to Ringtone.
• Go to Settings/Screen Time/Communications Limits. Tap each of the 2 entries and make sure both are “Everyone"
• Restart your phone-->RESTART your iPhone, iPad, or iPod touch - Apple Support

Hi there Jshoelover,

We understand you're experiencing an issue with a short ring time when receiving a call, and we'd like to see if we can assist.

First check out the steps here: If you can't make or receive calls on your iPhone

For additional help with how long the call rings prior to going to voicemail, you may want to reach out to your carrier to check for any options on their end.

Regards.

That luxury however may only be available in UK/Europe.

Google your phone company for the dialled code to increase your ringtime to 30 seconds before diverting to voicemail

It will be a string of numbers, * and #