iPhone 11 Pro

That’s a general problem with speakerphones, especially when both ends are on speakerphones. Commercial standalone speakerphones like this one (https://www.dell.com/en-us/shop/poly-realpresence-trio-8500-conference-voip-phone-with-bluetooth-interface-3-way-call-capability-sip-sdp/apd/a9949661/phones-and-communication) can deal with it better, but you are talking in the multi-hundred dollar range just for the phone.

The problem is when you talk the “sidetone” - the sound at the other end - momentarily changes the direction of the call, so it sounds like you are breaking up. You can minimize it by keeping the volume as low as practical, and talking as close to the phone as is practical.

Hello,

Start here:

Get help with the microphones on your iPhone, iPad, and iPod touch

Then test with the Voice Memos app.

Good luck.