If I'm not going to make my telegram public, how will I receive the PM from unknown people who is looking for the service which I'm offering?
I included it in the OP. You can use the forum PM option as a means on contact from potential clients and if the conversation needs to be moved to another platform, you can then give then your other contact details, this way only a few individuals would have access to it and that is after they have already had contact with them, and can choose to trust them with the information.
Even if there is the possibility of scammers contacting you and posing as clients, I think it poses less risk than when the information is out in the public.
Afaik, telegram user name can't be clone unless if they will add a similar letter(s) and looks like your user name.
That's true, I just slightly updated the OP. I was lost for a better word to fit the description.
I dont think users who have a lot of experience on forums will easily trust someone here.
Those contacting someone for a job or service may not likely have lots of experience.