Since Android 10, Google has removed from the System Phone App the ability to initiate calls with the speakerphone already active; to get around this limit you need to install a third-party Phone App
that still has this functionality available.
You can search in the Play Store for Apps that have this feature or you can use the free NanoPhone App developed by Aldea.
It must be configured as the default App for the Phone function (Android settings->Apps and notifications->Default apps->Phone app).
NanoPhone was developed exclusively to keep the possibility of starting GSM phone calls with the hands-free from VerticalMan. Therefore it does not have all the common functions of a telephone app.
To make calls interactively, the user can continue to use the System Phone App.