Here's something we can try. This works best if you have a way to call into your voicemail and play back while your still in the car. Usually most VM systems will let you review your message or clear it and start over. If not get someone to test with you. If your VM is recording and your not speaking you'll hear a boat load of road noise on playback. To determine if this is coming in from the inside. Place the palm of your hand over the Mic and see if the playback is silent or not. If your still able to hear a bunch of roadnoise then the mic has got to be picking up a lot from the underside of the headliner. If it's totally silent then the source of the road noise is coming from the interior of the car and if thats the case your best bet is dynamat the sheetmetal of the roof.
I haven't had a chance to test this yet. Road my motorcycle to work today but when I get a chance to try it I'll let ya know what I find.