Is authorization allowed only by phone number?

Question

Computer networks

It seems to me that authorization only by phone number is complete nonsense, an attacker can not only steal my phone from me, but also take advantage of numerous holes in SMS and GSM technologies.
In addition, it is simply inconvenient, when traveling around different countries many people buy temporary local numbers, it would seem that the Internet should be the same everywhere, but logging in using your number (if you have access to the Internet but there is no roaming on your "native" SIM card) becomes impossible.
How can people use all these WhatApp, Viber, Telegram in such a situation?

Answer 1

I fully agree with Artem, which depends on the context.
As for Viber and other services, there the phone number is not checked after sending a confirmation code once. Therefore, it will work with another SIM card if you do not reinstall applications or clean their data.