How to stop phone from automatically decreasing media volume while in a call for IPhone 13 Pro Max

Hello andrexxxxq,

Welcome to Apple Support Communities. We'd like to have you try these steps to help with this volume trouble you're having:

1.Remove all screen protectors, films, or cases from your device. 

2.Check the speaker opening to see if it's blocked or dirty. On iPhone only, make sure that the receiver isn't blocked or dirty. 

3.If necessary, clean the speaker or receiver opening with a small, soft-bristled brush. Make sure that the brush is clean and dry.

4.Go to Settings > Sounds (or Settings > Sounds & Haptics), and drag the Ringer and Alerts slider back and forth a few times. If you don't hear any sound, or if your speaker button on the Ringer and Alerts slider is dimmed, your speaker might need service.

5.If you hear sound, try making a call with your iPhone and turn on speakerphone. On iPad or iPod touch, make a FaceTime call. If you still can't hear, or hear static or crackling, then your network or reception could be the issue. Try to call again later, or from a different location.

More info can be found in the Apple Support article: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support

Should this issue persist, get in touch with Apple Support for further assistance:

Apple Support

Cheers!