Short codes are so great, I am sorry to hear your phone is making it difficult. Not to worry, I am happy to help you out today!
There are few things which could be causing this issue for you. If you are using a 3rd party application to send SMS, you may need to adjust the settings within the app itself in order to the 1 not included. I know for Hangouts this can be a problem for some short codes.
If you are not using Hangouts and instead the default application, you may need to clear you cache and data within the app. For this you will want to head to Settings > Apps > Application Manager > All > Messages > Clear Cache. Once cache and data has been cleared, please turn your phone off and back on.
Our Troubleshoot and Resolve tool can also help out with issue like this. It is like have tech support at your fingertips whenever you need it. Once you reach the page, please choose your make and model device > Service/network issue > Messages. You would then follow the prompts to resolve the issue.
One last thing to keep in mind, if your phone is unlocked from another carrier this can cause some problems when trying to text. Some phones will automatically add a 1 in front of the number.
I hope this information helps! Please don’t hesitate to come back if you have any other questions or concerns.
Michelle, AT&T Community Specialist
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