Telegram is OK with alternative clients, so as long as you’re not doing evil stuff, using slidge to interact with the telegram network is fine. The plugin uses telegram’s official TDLib.
Contact JIDs are of the form
email@example.com where 123456789 is a telegram ID.
If you want to find the telegram ID of someone using their phone number, use slidge’s search feature:
Finding legacy contacts.
Your contacts’ puppet JIDs presence statuses will show when they were last seen online, and their presence statuses will be set to “away” after 5 minutes.