Echo issue when someone calls on my iPhone 13 mini running iOS 18.6.2

People who phone me, hear thelselves as an echo with ca. 0.5 sec. delay - very loud and clear echo.

If I use a headset or switch to speaker mode, the echo dissapears. I don't hear any echo on my side.

The phone is an Iphone 13 mini with the latest IOS 18.6.2 installed.

I could really use some good tips to get rid of this problem.

[Re-Titled by Moderator]

Original Title: People hear themselves as an echo when they phone me