Well, this can be done with a simple and bloody trick: Every possible phone number is generated by a script. You can use an existing database or create a database for it. Then the so mined numbers are added to the phone and checked automatically, if they exist in Telegram. The same thing can be done in WhatsApp. Afterwards, if you've mined several millions of existing accounts, sorted by countries, you can do with them whatever you want, adding to groups in read only mode, writing automated messages and so on. This kind of data mining can be done with a simple laptop, just let it running a few days... Well, even knowing how to make this, I didn't do it ever...