Can not accept an incoming call while already in a cal

Hello, I have iPhone 7, iOS 13.4.1.

While I am already in a call (initiated by me or received) I should have been ablw to accept a new incoming call. On my screen I press the right icon (pause sign and green phone) so I can put on hold the call I am currently on, and to accept the new call. After one second of accepting the new call, the calls shuts off and the old call is still on hold. I can u hold and talk further.

This did not happened! I could always accept new calls and even MERGE the calls!

I did not change a setting anywhere.

In Settings -Call waiting, the check is green/enabled.

I even restarted my phone and I asked people to call me while already in a call, and they told me the call does not connect...