Call your telephone company business office.
You didn't describe the call content -- if there is no one there, it may be from a fax machine. Your business might help with that.
There is a call trace feature available in some areas that can trace most calls, even blocked. There is usually a fee. The number is given to the phone company's investigative unit. Check your telephone directory if Call Trace (*70) is available and its terms and procedures.
I don't know why they don't publicize Call Trace, though it is in the phone book.
{TELECOM Digest Editor's Note: I think *57 is call trace and *70 is 'suspend call waiting for the remainder of this call'. *57 is not publicized, maybe because of the huge fees they charge for what they used to do for free (investigate people causing trouble for others on the phone.) Telco does not like to admit that subscribers are entitled to the peaceful, unmolested use of their phones. As often as not, using *57 gets you a ten or fifteen dollar charge on your phone bill and a letter in the mail a couple days later saying they cannot do anything to help you. PAT]