One some cars the speaker button turns on the voice recognition so you can access the hands-free menu or your phone directory.
If your phone is ten years older than your vehicle it may not be compatible with the system. Manufactures have a list of compatible phones.
As @Nevada_545 says you might have phone compatibility issues. I am not into buying the newest phone, have a Galaxy S3 which is pretty old, so if I am buying a new/newer car, I check the compatibility.
The sales men at the dealerships where I bought my last two cars were disappointed that I did not have any bluetooth devices for them to pair to the car.
I too have an manual solely for what used to be called the radio that has more pages than the latest library book. I poured through that book to find out how I could turn off the light to the big touch screen that controls the radio or displays tire pressure.
I wanted to be able to do that to preserve night vision for driving on dark rural two lane roads but found out you can’t. Apparently driving is no longer as important as the entertainment system.
I can dim out the rest of the dash but not the touch screen, or so the dealer says.
Seems to me that going off hook should activate the speaker, no need for a second button.