When I answered, the person at the other end did not speak. When I called back, the automatic message said "Thank you for calling. Please wait while we transfer your call" but then would hung up instead of transferring me to anyone.
These spammers will call you daily using an automated system, and when you call back and get an operator, ask to be added to the non-callers list. Also, they have trouble understanding English, these spammers. Even after supposedly being added to the non-callers list, they'll keep calling you.