There are two versions of that same model number modem. They are:
MT5634ZPX-PCI-U : This is the one you want.
AND
MT5634ZPX-PCI-U-NV : The NV stands for NO VOICE. You probably accidentally bought this one.
I saw your review on Amazon so I think you already realized this. Hopefully this post will help others.
I am just about to purchase the modem or possibly the new PCI-E version of it. May I ask you if you replaced the NV one? If so, how is the new voice one working for you?