How to send messages to people in Telegram using the list of phones with the lowest cost?

Question

SMS

There is a site where there is a form for subscribing to a future webinar. Visitors leave their name and phone number. More than 95% of people on these numbers are in the telegram (by number - several dozen). The task is to send a simple reminder message to them in telegrams to these numbers an hour before the webinar (at a predetermined date/time).

How do people do this these days? Is it possible for free (well, so that at least spam is not blocked or there are no consequences)? If only paid, what services are good for a reasonable price? Or maybe SMS-mailing will be cheaper in general? And yes, is it possible to somehow automatically determine which numbers are in the cart and which are not?

PS I myself am a programmer in C # and ASP.NET with experience, so I can do the bot if necessary, and set up some APIs on different services. I just don’t know where to start and where to dig - there is a lot of information, and time - a couple of days.