Why do my incoming calls go to voicemail with iPad nearby?

Check to see if you have a Focus turned on there and have it set to share across devices. You can make it to where when you turn on Focus on one device, it will share on all devices , and it could be that it will do that when you get close to the iPad, if it is in proximity with the phone.

I'm just guessing here, but it is something to check. The switch is in Settings>Focus

Edit…

“why do my calls go directly to voicemail: My incoming calls go directly to voicemail if my iPad is near”

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Troubleshooting Calls Going to Voicemail:

Contact your Carrier:

Try the following. If no success, contact your Carrier.

A. Turn Off Do Not Disturb(DND):

Go Here: How to turn Do Not Disturb On or Off on your iPhone - Apple Support

B. Turn Off All Focuses:

Have any focuses? If so, turn them off one-at-a-time, until this no longer occurs. Use Siri, saying something of the sort: “Turn off the [focus name].” Learn how to use Siri.

C. Go Here: If you can't Make or Receive Calls on your iPhone - Apple Support

D. "No Service":

Is "No Service" shown on the top-left corner of your screen? If so, go here to fix that: If you See No Service or Searching on your iPhone or iPad - Apple Support

E. Install a Cellular Update:

It might be prompting you for a cellular update to install. So, go here for that. Update your carrier settings manually on your iPhone or iPad - Apple Support

F. Update the iOS Using a Computer:

• Use my User Tip:  Update your iPhone Using a Computer - User Tip
• IMPORTANT: If you have an option to update, however, then be certain you backup your iPhone first. That way, you will have something to restore your iPhone from should anything go wrong with the update