Right from the T-Mobile International Roaming FAQ page:
Unless you switch your device off or activate Unconditional Call Forwarding on your device, you will be billed for calls delivered to your voice mail box while you are roaming internationally.
Not only that, when I called T-Mobile to activate international roaming on my line, the rep told me about the charge and recommended I keep my phone off or do unconditional forwarding to minimize charges.
It's not a scam. This is how EVERY carrier works. If they have to send an incoming call to your phone while on another network, they have to pay that carrier and in turn, charge you. Thus, activate unconditional forwarding and calls never go to your phone, and you never get charged.