Since they only have a limited number of phones to call you on, just put them into your block list and move on.
If they are using "company phones" just call the company HR department and let them know. Be calm, don't flame to them and then block those numbers.
I use Nomorobo and my phone companies block list.
I work with 2 individuals that do not care for each other. One I like and the other I want gone. The one I like keeps receiving vulgar prank calls 10 times per day. I am suspicious that the other guy is using an app that is designed to harass this person by prank calling him and recording the call. How can I prove it? I do have access to both of their phone records as they are both using company phones.