I had two calls, the quality of one call was okay (the other person was on mobile data), the other one was crystal clear (the person was on WiFi).
I didn't notice any latency on both calls.
Please note that these features might not be final yet. They also might take some time until they are released, I'd guess load management is hard when more than 100 million people suddenly start testing calls.
WhatsApp took almost a year between their announcement to implement calls and the release. We all know that Telegram has the best and hardest working developers (😘), they are usually a lot faster.