>Telegram allows users to hide their phone numbers from strangers, but using your number as a relay could allow them to look up your Telegram account

You can disable discovery by phone number as well

>Then there is a massive issue of privacy, which allows strangers to look up your number and use it for spam and fraud.

They have info that this phone number exists and nothing else. Ok, also that the user uses telegram. Not much. This info can be reasonably used for spam with lots of numbers, not 150/month

Also I already trust sms to the centralized greedy third party usually acting against my interests called mobile phone operator. Now this also includes some random person and it suddenly becomes "privacy nightmare"