It's really reliable. Telegram's infrastructure and fault tolerance are among the best in the world. To establish calls to Telegram's central servers, only a short call is made, after which the call is routed through the relay-server infrastructure, or goes directly p2p-connected. Algorithms for the transmission of voice calls for Telegram are adapted even to work with weak and unstable Internet connections.
The infrastructure of SIP.TG servers for "landing" calls to virtual PBX has already been deployed in data centers in Moscow, Frankfurt, Amsterdam, London, Washington, D.C., Tashkent, and Minsk, and can be scaled up quickly if necessary.
The service has been operating since 2019, during which time many scenarios of its use have been fine-tuned.