Q: How do I verify the number via a phone call?

Regarding phone call authentication, the procedure is as follows:

1. Choose phone call authentication
2. Wait for the call
3. Call comes for example from +35844451234
4. Call is disconnected almost immediately - this is expected
5. Enter the last 4 digits of the caller phone number, in this case: 1234
6. Click ok

So it's by design that the call is terminated quickly - you need to enter the last 4 digits of the phone number that called your phone.