1: Prevent ringer volume from changing with button changes
Go to Settings > Sounds and Haptics
Toggle Change with Buttons to off
Put your ringer on the loudest setting
2: Add the Old Phone ringtone
Go to Settings > Sounds and Haptics > Ringtone
Select Classic
Select Old Phone
Many find this to be one of the loudest ringtone options.
3: Consider getting an Apple Watch
If you still find yourself missing calls, an Apple Watch might be helpful because it will vibrate and ring right on your wrist.