using retro handset with iphone 6

Hi all

My mom is 92 and 2,000 miles away and I bought her an old retro handset to plug into the iphone 6 she has...

I have found this to be the loudest for her to hear me.....Lately, for some reason, she cannot hear very well through it. I have kept on saying she needs to turn the iphone speaker OFF, so my signal (voice) goes through the red thing and not the iphones speaker...Am I correct? She had a helper just now try to do it...but no luck..

We mainly facetime. Even on my iphone 13 the speaker icon comes up when I make a facetime call...but touching the button does not delete the speaker function...(just for practice to see what happens..0

i am confused...maybe the red thing is dead??? I don't know...

Any help out there???

thanks a lot