Signal uses your primary number, but if 2 people are using Signal, they can send private, encrypted messages (or call, or video call) without having their number appear in the phone company's call or SMS metadata. Any communication between 2 Signal users is completely private.
Your Signal contacts are separate from your phone's Contacts (which are read by many other apps).
The only vulnerability is when an SP's phone is confiscated.
Best bet is a burner smartphone, using Signal to communicate.
WhatsApp is end-to-end encrypted messaging, but the people you contact must be in your Contacts, and everyone (other apps & advertisers) can see them.
https://support.signal.org/hc/en-us/categories/360000674811-Security