You can research certain apps/services that can help with that. The easiest and most convenient thing to do IMO would be to block any unknown/private/hidden #'s etc which can be done by apps too by the way. You can always unblock #'s that you recognize. There are different ways for tracking the signal/source etc but it would require some decent technical skills, some ways may involve some surveillance/spy methods (may be legal/illegal depending on what and how far you want to go), can pay a security specialist to do it for your employer (they have expensive equipment etc). Pretty much shared several examples, can research more details if you want but i do not want to break any rules by giving specifics (especially do not want to encourage or condone any unethical/illegal activities or actions) therefore, you are responsible with what you do/want to do. Hope this helps!