got a disconnected call when answered

0000000000 calls me all the time and leaves me a message saying this is regarding an important business matter. they leave a message telling me to call 866-390-9935. i called the number at least 20 times consecutively but it either keeps ringing or it brings me to an answering machine telling me that no one is available to take my call and to leave my name, account number, telephone number, and a time when best to call me.