As of now moodle is not simplified its authentication process for students.

because there lot of complaints about signup issues. if user is not able to access his email then that will be headache.

so there must be a simpler way to sign in and signup. 

as i know truecaller provides api for authentication

Here is a process

1 connect moodle mobile api to truecaller api

2 user is asking for mobile number during signup process 

3 One time password  is sent to the provided mobile number

4 As One time password is submitted user signup successfully

Documentation is here https://developer.truecaller.com/

There are other methods also.

example 1 using Facebook Account kit for mobile number Authentication

https://developers.facebook.com/products/account-creation

Example 2: Using Firebase Mobile Number Authentication

 Discussion is here https://moodle.org/mod/forum/discuss.php?d=362773