If your phone works like mine did if they leave a voicemail message it will tell you what number it is no matter what, so I would say try setting your VM message to sound like you picked up your phone and leave your phone off for a day. If your phone doesn't work the same way, amybe if you push the callback button after they leave a vm? Usually it reads back the number it's going to call.
You might also look into anonymous call blocker, if you're lucky it might be free.
You could always try calling the cell phone company- ask for a supervisor and tell them you are recieving harassing phone calls and need to get a number from a specific day--they have access to every number that calls your phone. Don't be a dick and they might be more apt to help you (some cell phone companies charge a fee for this service fi they do it at all), but also know this probably won't work...
Otherwise...change your number....
No we do not!!!!