Ahh, it sounds like you are looking for our voicemail to text service that is $4 a month. It sounds like the error you are getting is when you are trying to add the service to your account. Can you try rebooting your phone and trying to activate the service again? Your phone needs a data connection to the T-Mobile network in order to enable this service. If it's still not working, you will want to contact us so we can have our care team look at why the service is not adding to your account.
I also have a simliar problem, but I seem to have a hard time sending a voicemail from the web browser to my e-mail.
After a few attempts the voicemail system is not reachable....