Recently I signed on Telegram using my TextNow number.
When I signed in, I was assigned the profile of an existing user! I have access to the profile of this user, including the contact list of the user, subscribed channels, and session history with geographical information.
Apparently TextNow reassigns unused numbers to other users, which is quite understandable as there is a limited amount of them. However I am very surprised that a supposedly privacy oriented service allows me to unintentionally access other users' profile and, even worse, to have access to the phone numbers of in the contact list this user.
I could set an additional password and become the sole owner. On the other hand, if I change the username, I will still be on the contact list of unknown people and Telegram will notify my new username to them.
It is a fact that, when you get a new phone number, you are not necessarily the first owner. Using an old owned number reduces this possibility, but Telegram would advertise your presence to old coworkers or occasional friends, who are not in your circle any more and you don't want to be.
Is there a sane way to use Telegram without giving up to your privacy and security? (I don't think that Skype has these issues). This accident and the resulting attitude of Telegram toward (two factor) authentication worried me. Considering that one can lost the ownership of a phone number (for a number of reasons), is it sufficient to add a second level of authentication to stay safe?