Now he is blocked, and my Telegram handle was changed to make it less easily guessable.
I would advise you to automatcially block all incoming Telegram calls, unless made by one of your contacts.
For extra privacy you can also delete your username alltogether so only those who know your phone number or that are in the same group as you can contact you first.
Allowing calls from anyone was definitely an oversight.
And has been corrected the minute after I realised I left that toggle unselected.
Removing the username seems a little bit hardcore, as I prefer not to share the mobile associated with that username, even if it into the one I regularly use and it is not linked to my ID as it is KYC free.