They exist. Services treat control of the number as equivalent to control of the account, and expect you to maintain that control.
Throwaway phone numbers are not a viable low cost or no cost alternative in most normal user signup scenarios, and they're implemented as a privacy invasive form of spam prevention for that exact reason.
Throwaway phone numbers are not a viable low cost or no cost alternative in most normal user signup scenarios, and they're implemented as a privacy invasive form of spam prevention for that exact reason.