In the beginning (when I just created my account) I was having the same problem. I don't have your type of modem, but still VM was coming in. After I played with voxalot settings like disabling web calling (!?), disabling voice massaging, then it worked. I'm not sure why but now I don't have any ports forwarded from the modem(router) to my pap2 and it is working just fine. I hope it will work for you too.
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Vbuzzer, Voxalot and Unlimitel - by Andrei Nistor