From what I understand, the attackers steal your number by gaining access to your phone carrier account.
They can gain access to your account either by finding your info in a data breach, or by phishing the account details from you.
That's why they say that you need to setup a strong password with 2FA for your phone carrier account to protect yourself from this kind of attack.
I recently watched the movie Running Man, and your quote reminded me that in the movie they have court appointed agents instead of attorneys, lol.