Currently the phone number verification is via SMS. Many users complain that they don't get the SMS message. Is it possible to send WhatsApp and Telegram message from backend server to users (preferably with an organization account)?
I checked their docs but found nothing for this (Telegram mentioned that a bot cannot initiate a conversation with users and WhatsApp Business just had a "contact us" form), I'm asking here to see if anyone has info or experience. Thanks in advance.
On whatsapp you have two options (at least):
Using the official API, you should use an Authentication template: https://developers.facebook.com/docs/whatsapp/business-management-api/authentication-templates/
If the official API doesn't match your needs, for whatever reason, you can use un-official clients. Such is the case, there's a high risk of getting flagged as spam for sending messages to un-known users. Then the option would be asking the end-users to request the code. If they are the ones starting the chat session, there's no reason to be flagged as spam.
The same method would be also valid for Telegram, if you ask the user to request a OTP from a bot, then the bot would be able to answer and send it.