I can definitely understand your surprise at not being able to send 5 digit sms. Did you have Customer Care put a content block on your account? This free feature is designed to prevent subscriptions that usually charge 9.99 a month. Unfortunately this feature can also prevent free 5 digit sms too. Here's are some great T-Mobile Support pages on content blocking and premium service subscriptions.
In order to be able to send 5 digit sms in the future this feature would have to be removed.