I think Skype was able to connect on more restricted networks because it directly connected to some predefined IP addresses, it didn't need to have DNS working.

I think Skype was able to connect on more restricted networks because it directly connected to some predefined IP addresses, it didn't need to have DNS working. I remember this because my home network had the DNS servers down but still Skype miraculously managed to get connected to its servers. Maybe this is the same situation but for Telegram?