iPhone doesn't use Wi-Fi calling when cell signal is very weak

I had a long conversation with an Apple Senior Adviser last evening which was useful but finally unsuccessful.

Here's my understanding of the state of play. Yes, there is a "dead zone" in phone service when a cell signal is present but so weak that the phone and carrier can't communicate effectively enough to (a) place a call in either direction (b) agree to use Wi-Fi calling mode. As a result, the phone reverts to standard mobile mode, which combined with the weak signal means no calls can be completed.

Compare this to a phone in Airplane Mode, where the cellular radio is turned off, the phone and carrier default to communicating over the Internet, and calls are handled successfully via Wi-Fi.

Stating the requirement is simple - close the "dead zone" gap - but in the existing cellular service architecture it's probably not easy to implement and may require changes by both phone providers and carriers.