Set number of Rings before voicemail picks up

Answers are above. Apple does not control this. Contact your carrier.

For example, AT&T allows you to choose anywhere from 1-8 "rings" before a call goes to voicemail.

That is completely controlled by your carrier. Contact them and ask them to increase the amount of time your phone rings (it's actually a time setting, not a number of rings setting on their end).

That option would be controlled by your cellular provider. Contact them, or consult their website to determine if changes are possible.

Extend I- Phone 15 Ring time

How do I change the amount of rings on my iPhone 15 pluse