firebase phone OTP authentication: Issues with Indian phone service providers?

[Lavanian Dorairaj]

I am a user from India. 

I have setup html+js code that first uses  recaptha  to stop bots and then accepts a phone number to send a OTP (authentication code) to that number. Once the user receives the OTP,  he enters that value in a given text box and gets a popup whether the number has been authenticated or not.

The code works well on some phone numbers, but on certain others I just do not receive the authentication (OTP) code, consistently. I suspect that this could be because of the user putting their number on the Do-Not-disturb (DND) list. Such phones will only receive OTPs from certified providers and not just any provider.

I suspect that this could be true because certain providers in India guarantee delivery (on payment) to even phones on the DND list (I have tried tested one such and their SMSes are delivered in a few seconds to ANY Indian number).

My questions:

1. Has any one else in India come across this issue?

2. Is it possible that Google uses an uncertified provider to send these SMSes for users in the free plan?

3. Is there any alternate code to send these OTP SMSes through a certified Google provider?

4. Has any one in India tried blending google firebase code with code from such (reliable) Indian SMS providers?