Majority of crypto users don't like to loss their anonymity on purchasing bitcoin. If you just like to use it as username why don't you just ask for username and make adding phone number as optional if they want better security of their account and to get notification.
However thanks for your quick response.
If we use basic usernames / emails, the costumers wallets will not be secured.
We can not send or touch users coins by design : we use bitgo and users would have to authenticate [OTP] to send BTC, that means we can not abuse costumers, coins can not be stolen in case there was a hack or by an inside job.
It also prevent users from selling theirs accounts, creating duplicate accounts, etc.
