What is the easiest way to authorize a person in a mobile application using the REST-API?

Question

RESTful API

I am writing a server part for a mobile application and wanted to ask more experienced people what is the best and cheapest way to authorize a person in a mobile application.
Django3 + DRF and Channels on asgi and Postgres base are used as a server.
Authorizing a person by sending him one-time codes to his phone is expensive (2.50 rubles per SMS is the most acceptable option that I found). By login and password - too stupid for a mobile application, even if you only need to log in once. Using mail is stupid for the same reason.
I suppose that the most acceptable option is to enter the application using a pin code, but I still do not fully understand how to implement it.
Tell me, what authorization methods can be used and how do they work?

Answer 1

- Reset call to the client, let him indicate the last 4 digits of the phone (120 x 5 = 600 rubles / month).
- SMS to a client, take a non-public tariff with an SMS package, I got 0.21 rubles per SMS.
- Write in any of the messengers, search by number.

Answer 2

why can't you just send a password (pin code)?