Telegram doesn't encrypt messages by default. The encryption that they do use is "homegrown" and as such it can't really be determined if it works or not.  Even then, the encryption that they do use can only be enabled for secret chats - which I am unsure is preferable for an official brand channel.

Source - http://gizmodo.com/why-you-should-stop-using-telegram-right-now-1782557415

Slack is good (despite limiting history) and there are a few good alternatives (such as Rocket).