How to enable authentication of new site members (SMS, instant messengers)?

Question

SMS

Hello!
We are making a new site focused on the English-speaking audience, i.е. site users will be from other countries.
Registration of users will be carried out, among other things, using SMS authentication (6-digit code).
Or, as an alternative, the option of using telephone messengers (Facebook Messenger, Telegram, Viber, WhatsApp) is considered - also a 6-digit code.
A prerequisite is free or almost free sending of messages to new users of the site (30,000-150,000 messages per month).
There is no great experience in this direction, and therefore a programmer with extensive experience in such matters is required.
Assistance can be methodical (consulting) or practical (providing an SMS gateway, software, etc.).
Payment is piecework, but within reasonable limits (the budget is not rubber!))).
Please write only to the point!

Answer 1

Sending free SMS, moreover, in such quantities does not exist.
To register via free SMS, you can use Firebase Authentication.

Answer 2

What will be offered on your site?
Why SMS authentication for users?
Why don't you want to authenticate via email? They are just free.