Telegram is giving away FREE Premium subscriptions! All they need from you is to use your cell phone as a relay to text out their OTP codes! And the recipient of the OTP sees your phone number! What could POSSIBLY go wrong with this deal?
PLEASE don’t use Telegram! I personally recommend Matrix as it’s totally FOSS, you can self host, there are tons of front end clients to choose from. Or even use Signal. I have my own issues with Signal, the fact they don’t allow third party clients, you can’t self-host, they have a proprietary shim in their stack that only they know what it does, they were pushing crypto, etc, but at least Signal is better than this garbage.
It lets that person know your number works. If that person happens to be a scammer or solicitor, you could run into issues.
People don’t need to get an sms from you to know your number works. There are tons of other ways including just trying to log in into telegram or Whatsapp with a list of many numbers.