How to actually save yourself in call/sms bombing?

Never pick up the phone if you don't expect a call. Even if you just pick it up or deny the call, the number immediately goes to a database with live numbers.

You’re welcome.

Not exactly the right sub for this but I'll still answer. Mitigation is the key here - using a Google Voice number or any secondary line for anything that isn't important or official. If the site isn't a bank, .gov or similar, it doesn't get my real info. For everything else, password managers. ETA: you'll learn a lot in r/digitalprivacy and r/privacy

Find a service that gives you burner email addresses or use a throwaway email address for signing up for things. Never use your real email address and never use your real phone number. You can use Google voice to get a new phone number or you can use a burner phone (that you pay monthly). In the industry we call them sock puppet accounts.

Turn off your phone