They would only know if you wrote them a letter. they cannot take messages for inmates not accept phone calls, you'll have to either write them a letter or use the InmateAid service for Letters or Postcards.
https://www.inmateaid.com/ask-the-inmate/how-would-an-inmate-know-if-am-trying-to-contact-them.amp#answer
Accepted Answer Date Created: January 16,2018