I too would be honest about money matters with a teenager. That's a beyond a good age to start understanding how money, working, expenses, etc, work.
Explain that child support is intended to pay for things like clothing, food, shelter, utilities and other basic needs. Extras such as university tuition, expensive hobbies and other optional stuff is handled individually.
Explain that the government has a system to calculate child support, and that it's based on a person's income, and how many children are involved. Explain the offset calculation if access is close to equal (40-60). And finally, explain that you are paying full CS even though it should be offset CS.
If they feel that a phone is a basic need, they should be asking their mother to cover all of it. If they feel that a phone is an extra that should be outside the CS system, they have to convince you it's necessary before you'll pay a share.
If the kid notices that one parent doesn't have as much money as another parent, they will learn that this is because there are differences in how the two parents earn money, and how they spend money.
A teenager either has already developed, or should be developing, the maturity to understand money management. If you don't teach it to them by explaining your own money management, how are they supposed to learn?
We do children a disservice by telling them not to worry about finances, or explaining only that it's personal. You don't have to reveal specific amounts; you can just say things like a third of my income goes to support you at your other parent's, a third goes to paying the mortgage and utilities here, and out of the last third, comes the food and car payment and gas and fun and savings, etc.