Hi Cherub7171,
Although the ringtone volume is up in settings, this might be due to the volume being lowered on the phone. Try calling your wife and as the call is ringing, have your wife press the volume up button. Does the volume appear to be turned up all the way on the slider that appears? If so, you can lock the ringer volume in and prevent it being changed with the side buttons by disabling Change with Buttons if you'd like. Below guidance is from Adjust the volume on iPhone - Apple Support.
"Lock the ringer and alert volumes in Settings
- Go to Settings.
- Tap Sounds & Haptics (on supported models) or Sounds (on other iPhone models).
- Turn off Change with Buttons."
Also, check if your wife is looking at the phone while you call. This new feature called Attention Awareness will lower the volume when the camera sees the owner looking at it. Turn Attention Aware features on or off on your iPhone or iPad Pro - Apple Support
She can also turn that feature off, and test, OR have the phone facing down when you test.
Kind regards.