Connecting AUX cable to IPhone XR- Does not work with phone call

I have an IPhone XR. At work, I conduct conference calls with people on the phone and people in a large room with a loud speaker and microphone. I connect my IPhone to the loud speaker/microphone using an AUX cable.

The people on the phone cannot hear me when I speak through the microphone. Now, when I play music through my IPhone, it can be heard through the loud speaker when connected with AUX cable. As soon as a phone call comes through, there is no sound through the AUX to the loud speaker.

So basically the AUX output works fine with music, but does not work with phone calls.

Is there a setting that I am missing to enable a phone call sound through the AUX cable ?

The person who was doing this before me was using a Galaxy phone and phone call sound through AUX loud speaker worked well.