M0nica
To hold a telephone conversation, other than a few quick sentences, with anyone, when other people are around, is bad manners, whether the speaker is on or off.
This is how I see it, I would not answer my phone at all unless it was someone I thought their call could be urgent. Then I would say I am with so and so can I phone you back or is it urgent. I would have my loud speaker on at that point as I can't her without, if it was urgent, I would excuse myself for the short period I would allow the person who is calling to tell me the problem.
My friend was here Saturday her step son in law phoned about something totally unimportant about a golf club booking, she spoke with him for 20 minutes, then proceeded to repeat the whole conversation to me afterwards, had she put the phone on speaker phone I would have know and could have participated, if that was not acceptable she should have phoned him back when she got home. I would never have a speaker phone call on the bus or anywhere else public. I quickly answer and tell them where I am and say I will phone them back later.
My late partner always chipped in if was speaking with a friend we both knew, they appeared to like it, but I definitely did not, He never did understand why. So each circumstance is different, but I think we can all agree that it is in the main rude to answer your phone when in company with others. It is equally rude to be constantly checking your phone for messages. My own son and daughter in law do that as well as a lot of friends, that really appears to be the norm now, definitely not forme.