Telegram api has a method client.get_entity(str), where str is the phone no. in format '+91xxxxx xxxxx'.
I can save all the contacts having telegram account on a google spreadsheet for you. I can also create a functionality to message them all at once.
I have recently worked on a similar project where I had to forward all messages from a chat to Google Spreadsheet and the complete process was instantaneous, that is, for each message there was an update on the spreadsheet.
I have my telegram api key with me and I can deliver you this project within 3 to 4 days time.
I also have experience with machine learning which will help me generate random phone no.s with maximum probability that the phone no. has been alloted to a customer by a telecom company. the machine learning model will be trained with information given that the telecom companies are alloting phone no.s in a given range for a given country. ( like for india some series are like +918275xxxxxx, +919423xxxxxx, +919028xxxxxx, +917558xxxxxx, +917557xxxxxx etc.). I will make a list all the series for a given country and then generate random no.s from them or I can make list of all the phone no.s that are to be existing and sample this numbers to the system to generate random phone number. whatever time I will take will only be in generating this random numbers correctly and rest of the things won't take much time.
This are my ideas, please contact me if you want your project to be delivered as fast as possible.
Thank you.