How to make iPhone Longer ring when receiving an phone call

That would be an option that would be offered by your carrier.

For example, with AT&T you can set the phone to ring 1 time.....(6 seconds).... up to 6 times....(36 seconds)....before the call goes to voice mail.

Check with your carrier to see if they offer a similar option.

The carrier limits the number of rings. Check with your carrier, and ask them if they offer a means to change this.