Why the accounts of the people working in Telegram are not banned

Telegram applies the same rules to anyone, irrespective of the person. The only reason why the accounts of Telegram employees isn't banned is that they can't get a new number every time (Telegram's secrecy) and that they don't do any such thing to get a ban. They're private and don't contact anyone again and again, either simply or to test if a problem is solved. As they have access to then support and moderators, they can just send them a message and get their problem solved. They need not test it to see if it's solved because if they come across it again, they can ask support and it will be solved in seconds. Therefore, they don't break any kind of rule, as such to face bans. As simple as that.