Set number of Rings before voicemail picks up

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).

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 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