Why is it impossible to increase iPhone rings before vmail forward?

Not joking. My mother has an iPhone and it rings maybe 3 times then goes to vmail. She can't get it out of her purse fast enough. I thought no problem to increase rings before that. No settings, no options at all. AT&T makes no mention of it on their support site other then to have a support rep do it. I had three different reps (adv tech) swear to me on three different calls that did it for her and all failed. They have a setting that allows up to 40secs before forwarding they can configure I suppose. Each of them did not work. I did various #61 carrier programming ideas around the Internet but it's insane this is impossible.