I don't know if you still need help, but since this came up in a search I'll post an answer.
You can find a little info here: Message Blocking
Here are the steps that I followed and I was able to SEND a message.
Go here: https://my.t-mobile.com/Login --> Go to --> plan --> select user that you would like to change --> change services --> uncheck the message blocking service --> hit next and confirm.
Mod note: If for some reason these steps didn't work for you, create a new post so we can get you answers right away.
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Its a bug in the system. I also chose to block downloadable content. After I did that, I wasn't able to do text bankiing with Chase to check on balances, etc. However I continue to be able to text everybody else including other banks such as Wells Fargo to get balances, etc. So maybe Chase Mobile (24273) is somehow mislabeled as a source of downlable content thru T-mobile. I don't know.
Anyway, after I unchecked the block downloadable content, I was able to text 'Bal' to Chase Mobile to get balances, etc.
This worked for me thank you, lol found this post a year later
In my case, it meant that I did not have funds to send SMSs. Normally text are free as part of a pre-paid plan, but not if you are out of the country (depending where I suppose). You may have also hit a limit. I added money and then texts started working again.