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.
Signal is fine for a drop-in WhatsApp replacement. I use it for chatting to my friends casually. For something you need more security for you could do encrypted emails as that doesn’t require exchanging phone numbers, or ideally just arrange to meet up in-person and discuss things so you don’t leave any kind of digital or paper trail.