Sprint uses a few different 4-digit short codes for free messages and self-service account management. Below is a list of some of the short codes, I will try and keep this updated as new codes are added/removed for use.
9099 - Sprint Free Msg
This code is the default used by Sprint to communicate information about an account.
9999 - Block Text
This code allows you to block/unblock text messaging to specific numbers. You can text HELP to get a full list of commands. LIST will give you a list of currently blocked numbers. BLOCK (then the number) or UNBLOCK will allow you to add/remove numbers from your block list.
9000 - Picture Mail
This is the short code used for picture messages on older or simpler phones that do not support full MMS.
1311 - Usage and Upgrade Eligibility
This short code allows you to request your upgrade eligibility and current usage information (minutes, text and data). Text UPGRADE or USAGE.