Yeah, but on the website it says

You can activate WorldClass International Service by calling 611 toll-free from your T-Mobile phone, or calling 1-800-937-8997 from any phone.

There is no recurring monthly fee or activation fee for this feature.

Please note:

Only the "account owner" may request activation.
Prepaid and SmartAccess customers are not eligible.

(color added)

I think it's pretty hit or miss if they will allow it or not, I know for my prepaid it was not an option. I would not have that be my only source of service until I was sure it worked first by trying it someplace.