There are some important regulations parents can take to make sure their kids use cell phones in a safe and responsible way. When kids know what the consequences are when rules aren't followed, that makes life more predictable for them.
Most kids are not paying for their cell phones or their cell phone plans, so if you pay for it, you own it, which means you have the right to take it away anytime if they disobey the rules you set up earlier.
After a kid has a cell phone, suddenly parents discover data runs out half way through the month. This can get expensive and your kids need to know how to monitor their use. Also, teen cell phone addiction should be limited. Generally, it's a good idea to have no cell phone policy while they are doing homework and at bedtime. Come to agreement in advance about what they are and aren't allowed to download on their cell phone. There are some really bad apps for kids that you don't want on their phones.
Make sure your kids understand how to set up their privacy. Talk with them about why guarding their privacy is so important. Many teens truly have no concept about everything that a cell phone can share about them including personal information and their location.
You want to cover what is and isn't appropriate to share via a cell phone. This definitely means photos and language used in emails and on social media. But don't forget simpler things like providing their address, their age or what school they go to online.
Lastly, there are times and places when using one's cell phone is not appropriate. In movie theaters or during class time at school the phone should be on silent and put away. Additionally, when someone is having a face to face conversation with them, they need to be looking at the person, not their phone.