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I want to know how to change the amount of rings the phone will ring before it gets sent to my voicemail.
You certainly cannot do that as part of the iPhone's functionality.
It's possible your cellular carrier can do that for you, but I'm not certain -- I sort of doubt it. But anyway you should ask them. It's your only chance.
I should add you can do "Do Not Disturb" from your iPhone which will, in some sense, change the number of rings (you will not hear any rings).
I assume that is not what you're trying to do.
You have to contact your carrier, it can be configured on their end.