Using Voice Control during phone calls in iPhone

First of all, I'm using Siri and also Voice Control (the accessibility feature) regularly. From time to time, while I'm on a phone call, I get a weird notification of "***to use voice control during a call enable Siri***". Why to enable Siri if it's already enabled? Secondly, when Siri is enabled, **how can I use Voice Control in a phone call**? And why do it need (seems to) Siri for that?