We are getting "account locked" when trying to test SMS delivery from Sandbox environment. Using the out of box external account - configured to NetSize.
Does the error mean, account with NetSize has been locked?
I am not aware, if any. I'd recommend getting in touch with support to explore the options you have to test SMS with the Sandbox.
I'll mark this thread as answered as the original question was about your specific "locked" error.
I am just exploring the product in Sandbox instance. Is there any demo account I can use for SMS? Any pointers regarding this will be helpful.