Phone message: nai sen 2 shou chou

My phone has been ringing off and on for a while and when I pick up there's a tone like that for a fax coming in, but then nothing. Phone message display shows (katakana) nai sen 2 shou chou.

How to translate, please?

"nai sen" sounds like "extention (number)," it must be from an office, where they have their department/section names and numbers registered on their fax/phone memory. But you cannot really do anything with it - apparently someone has mistaken your phone number for another fax number, and trying to send a fax message to that number!

If it comes in too often and you want to stop it, I believe NTT has a service (I believe at a charge) that after you get a call, you call up NTT to block the last number that tried to call you...