This case has been resolved. You should now be able to send SMS again.
Original Message:
We’ve received reports that our U.S. shared numbers are not able to send outgoing SMS text messages at this time. If you are located in the U.S. and do NOT utilize the Platinum Plan, please refrain from sending out SMS messages until further notice is given.
We are in active communication with our SMS provider to get a solution in place. Any updates will be sent via the Support Blog.