The first call to my iPhone is always "busy"

Hello, fellows! have a really troubling problem with my iPhone 7.

The first call to my iPhone 7 usually provides response "busy" to the caller, while it's not!

The caller gets a "busy" response while I get a "missed call" @ my phone.

Immediate redial makes a call pass as it should always.

Mobile operator implies that the problem is with my phone, not on their side.

Can someone please tell me if there's some setting that could make my iPhone 7 behave so strange? It really provides me troubles - some partners think that I added them to black list.