[postmodernbrute]

You can cloak your phone number & not allow others to reach you via your phone number. Which means that the authorities cannot match your phone number to your Telegram identity, even if you posted in a public chat. This feature was implemented last year during Hong Kong protests to protect against government efforts to identify protestors by enumerating the limited phone number space in HK. There is also a password option to protect against SMS surveillance.

As to whether Telegram itself would cooperate with data requests from your government - that depends on which government it is, and in the end is up to personal judgement. I don't think there's any reason that Telegram would betray me to the Chinese government, for example, while I won't at all trust Facebook for that. Facebook, and Zuckerberg himself (for how long did he stick to that Wuzhen avatar?), tried hard to appease the Chinese government for such a long time.

International megacorp are generally the worst to trust in that respect. To many of us in authoritarian places, the illegality of Telegram is itself an attraction.