They acknowledge this in their blog post: https://telegram.org/blog/15million-reuters https://telegram.org/blog/15million-reuters It's part of their contacts A

They acknowledge this in their blog post: https://telegram.org/blog/15million-reuters https://telegram.org/blog/15million-reuters It's part of their contacts API, where you submit the numbers you have in your contacts list and they let you know which numbers already have a telegram account. They have since added rate limiting to prevent brute forcing it, but it sounds like the API itself is still available.