I think people are confused as to why there is a speaker option on cell phones. It is there so that in the event of you being in a loud environment and still needing to be on the phone the other person can be heard clearly, but more so that in the event that multiple people in the same location need to, can participate in a conversation with ease. I will also accept it as a hands free option when you need to keep both hands working and be on the phone as well.
If you are one of the many people who walk around with their speaker function constantly engaged to have a conversation so be it, but please remember that you have now entered in a shared social contract with everyone around you. If I can hear both sides of the conversation and we are standing in shared public space then you have made me part of your conversation as well whether you like it or not. On more than one occasion I have added my “two cents” into a conversation only to have the user of the cellphone stare at me.
To make it clear your expectation of privacy is exactly zero once the speaker function is engaged. If you want an “A & B” conversation don’t invite C through Z. For the most part no one wants to hear your conversation anyway. There are two solutions to this dilemma here.
First, don’t use the speaker function. Most cell phones even going back a couple of years have superior audio technology. Hearing the person on the other side shouldn’t be an issue. Second, should you have hearing issues or need to be hands free there are countless devices to help with either issue. Wired or wireless (Bluetooth) whatever tickles your fancy and/or have the budget for.