Each parent and each child is different with different circumstances. Our oldest boy got his first phone when he was ten. That mind sound like to young but he had massive restrictions, it was mainly because he played baseball and was away for tournaments and we wanted to make sure he had a way to contact us if things were running late or in case he got injured. On more than one occasion we had to meet him at the hospital for x-rays and casts.
A flip phone was out of the question because our carrier didn't support that style phone, so smart is all each of us have. The key in our case was he or none of the other kids "own" their phone, they belong to me and the wife, they just get to use them and we have taken them away from time to time as punishment or for improper using, during restricted times. The wife monitors the kids phones so she is on top of them about how and when they can use them.
When I was a kid there were pay phones about every 3 or 4 blocks so we always had a few dimes in our pockets to call home if we needed too. You would be hard pressed to find a public pay phone now on any street here, those disappeared as the personal cell phone costs came down.
Each situation is different and needs to be treated that way in my opinion, but monitored with restrictions has always been a part of our kids phone usage.