You currently need a phone number to sign up for Telegram. Add an option to sign up using an email address or another method, like some messengers do (e.g., Wire, Matrix, Threema, Session).
Potential spam issues could be mitigated by:
Avoiding an identifier tied to your identity, hiding identity from governments or people who know your number.
Use a virtual phone number.
Warning: Do NOT use temporary numbers to connect to your account, as you may lose access to your account when you lose access to the number.
Log in here to report bugs or suggest features. Please enter your phone number in the international format and we will send a confirmation message to your account via Telegram.
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In any case even if it won't resolve all the problems related to crypto there are a few one, just like Monero, that are focused on privacy and security (and in that case Telegram should incentives them).