Hm can it be estimated / is there public information about how many phone numbers are taken? E.g. I generate a valid number for one country or state, how likely

Hm can it be estimated / is there public information about how many phone numbers are taken? E.g. I generate a valid number for one country or state, how likely is it that the number is in use or registered? I once got a phone call from a university student for a survey for their project and they told me they generate them randomly which makes me really wonder, how likely is it?