noun: contract/ˈkäntrakt/: a written agreement
“Agreement” is not always the case between parents and children about their phone.  “Conflict” is much more common.
Help prevent future conflict by making a cell phone contract with your child.  
A contract forms a foundation which can be referenced later if issues arise, and ensure’s parent’s expectations are clear.
Below are a few contracts to use as a starting point to develop one that fits your needs. The third contract can be modified in MS Word.