Can't you access other people's number from telegram? I thought you could (since it's required).
No. You can´t know the phone number from a user on telegram. You could search for him using only their nick.
Maybe not through traditional means. If the information is out there, it can be accessed by someone. When working in the financial industry, I was always shocked at how easy it was to access client SSNs or other personal information. That, and social engineering is typically a fairly easy way to get information like this out of nonchalant employees. Way off topic though. Sorry.
You'd have to explicitetly share the account name with someone else, so no -- at least through what Og said, traditional means.
But yeah, most companies (in many industries) have some pretty horrible 'security', or for this matter a lack thereof, when it comes to all of your most sensitive information (tax returns, id scans, ssn, etc)