SelfDugGrave.13

Download "Signal" on Mobile: https://signal.org/download/
Download "Molly" on Mobile: https://molly.im/

Singal can be connected or "linked" to your PC once you are set up on mobile.

Singal required a mostly valid phone number at least during the verification process.
If you're using a burner number, you shouldn't need it again post-verification. Unless you delete the app, and need to sign-in again. It'll ask you for a 4-digit pin, which is all you need later on, should you lose your phone number.

Don't want to give your phone number to yet another entity? Consider "Delta Chat."
https://delta.chat/
No phone number required.
No username required.
No email required.
It's pretty simple, at the cost of a simple and basic chat. Which means no rich text formatting.