So if we don't use the real phone numbers for our telegram, then which phone number we use
This is making me confused.
What I meant is, don't use your main mobile phone number as you usually have to leave ID info in order to get one (at least that became standard practice in here in EU) and instead use virtual mobile phone number service to create Telegram account if you can't get another sim card without giving your ID. Well, at least that's what I did 6 years ago when I got Telegram for the first time. Trust no one, including Telegram.
What do you mean that service is in demand? This service does not seem to be legit and only scammers or hackers may be willing to get this data. What is the use case for this data?
I could see it being used by marketing agencies, spamming their crap to Telegram as well.