Every call you make on any cell phone to any other cell phone is already encrypted. Problem is, it is very easy to break that encryption (criminals, police, and governments do it continuously).
I’m guessing your actual question is “how do you securely, end-to-end encrypt calls from one cell phone to another”.
For this, you will need to use an app on both phones, instead of using the built-in voice service.
The two leaders in this market are Signal and Wire. My personal preference is for Wire, due to cost (free for personal use), and multi-platform support. Signal is an excellent product as well.
It should be noted here that the situation is even worse with Voice Over IP (VOIP) phones, and simply dismal for landline calls. NEVER use either of these to discuss anything sensitive.