[Fradow]

First: I used "Calling apps" because that's how it's displayed on my Android 8 phone. The actual naming isn't consistent across Android versions, so yours is probably named "Phone apps". That's located in the settings, and again the exact way to access it varies according Android versions, manufacturer and whatnot (which is an endless source of pain to guide end-users by the way).

To appear there, apps have to declare they are phone apps and handle the proper calls (an "Intent" in Android jargon) when the system receive a request to make a phone call. WhatsApp, Signal and Telegram do not do that: you are only able to initiate a call when already inside the app.