Not free, they are lying

I get numerous calls from this number on my extension at work...I don't know if it is coming in on my direct dial line or being sub-routed through our phone system.  It drives me crazy because there is no message, just the call probably 5 times a day at different times.  I usually come in each morning and have to clear at least 2 messages from my voicemail, and then throughout the day, I look at the  number on my phone display and see that it is the same number.  What can I do to stop the calls?  

called them back no ring, phone system picks up and announces as ERC. has option to report wrong number but could be a verification trap