There is never any one on the line. They have called me on my cell phone about 8 times.

hung up on my cell phone voice mail

Have received 7 calls from this number in the period 11/7/16 - 11/18/16.
If I pick up, there is no acknowledgement from the caller.
When I call back to ask them to cease and desist, I am prompted to enter the extension followed by the pound key.
Attempted to enter multiple random extension numbers w/o success and was disconnected.
My phone is on the "do not call" registry.  Shouldn't this be illegal?