I don't let them answer the phone. It's just not anything they need to do. The phone calls are for me or my husband and not them. When they are around 10-12 and have friends that want to call them we will probably get them a teen line so we can keep our phone for our calls. The family I was a nanny for had a kids line and it was upstairs, all 7 kids had it for their calls so yes there were some sharing issues. Also, if no one was upstairs the phone could ring off the hook. They had the phone wired from the bottom of the stars and could unplug it if needed too.
With so many people only using cells now it often annoys me when I am calling a friend, especially calling from my cell to theirs, and their kids answer and are playing a game on the friends cell and don't tell them I am calling. It is wrong on so many levels. I just wasted money on a daytime phone call and they are playing a game...It annoys me, can you tell?
I guess we tend to revert back to our parents attitudes when it comes to things like this too, my parents didn't let me answer the phone so that may be where I get the base attitude.