All our kids at home know we have Qustodio on their phones and can see everything they do and control access to specific apps and time limits. We have their passwords and they know we check their phones regularly. Their phones also go to bed in our room at a set time each night.
They can earn social media accounts and eventually mobile service by accomplishing significant things, like learning to play the piano, finishing personal progress/eagle scout (going to have to adapt to new programs), etc.
We openly discuss with them that this is meant to ensure they are mature enough for social media accounts and "always on" access to their phone. It is not that we don't trust them. Some things are better with more maturity. I give them real-life examples.
This approach also gives them some control over when they get it. If my daughter brings up an Instagram account or mobile access longingly, I can say, you know what you need to do to earn it. Let's go figure out what you need to do next on personal progress.
Our older kids had a gameboy, mp3 player, and digital camera by 7th/8th grade. Our kid's wifi smartphones have now replaced all that in a modern way. Google Voice is nice because they can still text over wifi and not be a social pariah. It also has a voice dialer for calls, but kids hardly make calls these days.