This same thing happened to me with Discord once, I just wrote to them and filed a complaint saying my account got "hacked" and that the hacker changed the 2fa mobile number.

With adequate proof and justification I think you should be able to get back your accounts, that is if your service does allow you to get back your account. One tip is to have multiple phones for 2fa and then just not using the other phone, if you don't use it how can you lose it? (that's what I did since I lost it once).