I get similar calls. People call, try to disguise their voices, make threats, etc. Or just call at 2-3 am to wake me up. Your phone company can help in these matters. Just ask them.
There are very few real coincidences in life... I say NEVER assume anything is a coincidence. Check it out thoroughly. Learn all you can about this incident, because knowledge is power. Some people call this kind of thinking "paranoia" ... I call it just being cautious.
I would never, ever answer again to that number. Someone might link you to the offender and name you as an accessory/conspirator if that offender is under investigation for anything. Cops have to be especially careful about who they associate with. You could possibly be made to look like you are communicating with... "A KNOWN SEX OFFENDER" if someone is after you. Can you see the news headlines?
If it persists, notify your supervisor, you might even want to do it now, and start a trace.
I personally never answer an unknown/id resrticted number, or even a number I don't recognize, just to be on the safe side.