Finding an unused number isn't going to save you for long. Every account you make with the new number is waiting to be leaked in a future hack. Furthermore, all your contacts are probably sharing their contacts (you included) with every app that asks for that permission.

There's only 7,970,000,000 possible phone numbers. Not even enough for everyone on earth to have one. And an advanced robo dialer swarm can call them all in a day.

If you're in the USA, there's a national do not call list. https://donotcall.gov

After opting out, wait a few weeks, then you have the right to sue any unsolicited marketers who call you directly.