Investigation by investigative journalism outlet IStories (EN version by OCCRP) shows that Telegram uses a single, FSB-linked company as their infrastructure provider globally.

Telegram's MTProto protocol also requires a cleartext identifier to be prepended to all client-server messages.

Combined, these two choices by Telegram make it into a surveillance tool.

I am quoted in the IStories story. I also did packet captures, and I dive into the nitty-gritty technical details on my blog.

Packet captures and MTProto deobfuscation library I wrote linked therein so that others can retrace my steps and check my work.