There may have been a time when you called in to get support on your phone and they had you type in some random numbers under your phone to set something up. These codes are called USSD and there are a number of them that developers use to troubleshoot problems and configure devices without having to contact the cell company for support. Below is a list of the known supported codes.

This entire post is copied from an XDA thread. I posted this strictly for information and educational purposes only. Use at your own risk.

If you post them I will update this with a list of all the ones that work.
Some codes from i9300 do not work. Don’t post any not tested or not marked as dangerous.
Most of the Android secret codes only work on Samsung stock rom, the dialer recognizes the
code when entered and you don’t have to hit send. Codes work on ICS and JB dialers, but
most are not built into CM10 or AOSP
(if you erased any of the stock /system/app/*test*.apk obviously they won’t launch)

[UPDATE — There is some hysteria on the interwebs about exploits that can reset your S3.
All that is happening is certain tel: links in a browser or NFC are loading numbers into the dialer
for you. When any Android Secret code listed below is entered into the dialer it launches
the specific .apk or function (like reboot or even factory reset). You can install a 3rd party
dialer and set that as the default and avoid this. The codes do nothing in most non-stock
dialers. They also are present on almost every phone, not just S3]

USSD Codes (T-mobile)
#BAL#
#MIN#
#MSG#
#SMS#
#WEB#
#NUM# — returns your phone number
#FAM# — Family allowance
#PWD# — reset voicemail password to last four digits of phone number
#PWO# — toggles voicemail password
#RON# — enable international roaming
#ROF# –disable international roaming
#TAXI — calls a local taxi service adds $1.75 to your bill (US & Canada)

*NUM — reads your calling number to you
*777 — Smart Access Customer Care
*TOGO — Prepaid Customer Care add minutes
*ADD — Prepaid Service Center add minutes
611 — Customer care
123 — Voicemail

VSC/ MMI Codes (GSM)
Other GSM Supplementary Services codes
*#30# — status of incoming caller ID
*#31# — status of outgoing caller ID
*#35# — status of incoming SMS
*#351# — status of Incoming SMS If Roaming
*#33# — status of Outgoing SMS
*#331# — status of Outgoing International SMS
*#43# — status of call waiting
*#21# — status Unconditional Call Forwarding
*#002# — status all Conditional Call Forwarding
*#004# — status Conditional Call Forwarding (busy, no answer, unavail)
*#61# — status of forwarding when no answer
*#62# — status of forwarding when not reachable
*#62# — status of forwarding when busy
*xx# — enables feature
**xx# — register and activate feature
*#xx# — check status of feature
#xx# — cancels feature
##xx# — cancel and deactivate feature
ie.
*61*1xxxYYYzzzz**10# — sets no answer after 10 seconds to forward to (xxx)YYY-zzzz
#31#2345678910 — initiates call to (234)567-8910 with outgoing caller ID blocked
*43#2345678910 — enables call waiting and dials (234)567-8910
#35*0000*16# — enable SMS service (where 0000 is default Network Barring password)
*35*0000*16# — disable SMS service (block incoming SMS)

Android Secret Codes (SGH-T999)
Universal
*#06# — IMEI

Samsung Dialer – [Source] [here] and in thread
*#0*# — SensorDiagnotics [1]
*#03# — NAND Flash S/N
*#80# — Touchscreen noise test (warning do not press power button)
*#251# — Wakelock ON
*#250# — Wakelock OFF
*#350# — Reboot
*#526# — WLAN Engineering Mode
*#528# — WLAN Engineering Mode
*#745# — RIL Dump Menu
*#746# — Debug Dump Menu
*#0011# — ServiceMode [1]
*#0228# — Battery status
*#0283# — Audio packet loopback test
*#0289# — Audio/speaker test (Melody Test Mode)
*#0514# — SysDump (System Dumps, logs, etc)
*#0588# — Proximity Sensor test
*#0589# — Light Sensor Test Mode
*#0673# — Audio/speaker test (same as 0289)
*#0782# — Real Time Clock Test
*#0842# — Vibration Test
*#1575# — GPS Control Menu
*#2263# — RF Band Selection
*#2663# — Touchscreen FW and version
*#2664# — HW touchscreen test
*#3264# — RAM CHECK (Version number)
*#4327# — HDCP INFO (True/ false)
*#7769# — PROXIMITY SENSOR TEST
*#7284# — Phone Utility (USB Settings)
*#7353# — Self Test (Menu of 12 tests to use)
*#7594# — Factory mode Enable shutdown on long press
*#8888# — Service Mode (Read FTA hw version)
*#9900# — System Dump Mode
*#9999# — SW Versions (AP, CP, CSC, FTA SW)
*#22235# — ACCELEROMETER SENSOR INFO
*#86824# — TOUCH KEY SENSITIVITY
*#5238973# — LCD TYPE
*#232331# — BT test
*#232332# — ???
*#232337# — BT address
*#232338# — WLAN MAC address & PS mode
*#232339# — WLAN Engineering Mode test (TX & RX modes) (same 526)
*#272886# — Auto Answer Selection
*#3214789# — GCF Mode Status
*#8736364# — OTA Update Menu
*#7412365# — Camera Firmware Standard (Front/Rear Cam FW)
*#7465625# — Personalization (Lock Status settings)
*#22558463# — Reset Total Call Time
*#12580*369# — Main Version (AP, CP, CSC, RF cal, HW Rev) (similar to 9999)
*#197328640# Service Mode
*#3282*727336*# — System and Storage Information (Data Usage Status)
*#273283*255*3282*# — Adjust/Fill Message, Contact, Call Log, etc Limits
*#273283*255*663282*# — Data Create SD Card
*#7328735824# — LOT ID

//*#7780# Factory Reset (WARNING)
//*2767*3855# Full Factory Reset (WARNING)
87976633 FACTORY RESET

unconfirmed (may be i9300 codes)

*#32489# — (Ciphering Info)
*#4238378# GCF Configuration
??*#872564# USB Logging Control
??*#3214789650# LBS Test Mode
*#34971539# Camera Firmware Update
*#7412365# Camera Firmware Menu
*#301279# — HSDPA/HSUPA Control Menu
73876766 SELLOUT SMS ENABBLE
738767633 SELLOUT SMS DISABLE
7387678378 SELLOUT SELF SET TEST MODE
7387677763 SELLOUT SMS SET PRODUCT MODE
4238378 GCF SETTINGS
4387264636 SELLOUT SMS MAIN
*#528# — ?? Wireless Lan Tests
*#4636#
*#7465625# — unlock? errors?
*#197328640# – Enabling test mode for service activity
**05***# – Execute from Emergency dial screen to unlock PUK code
??*#07# Test History
*#1234# View SW Version PDA, CSC, MODEM
2886 ??

Also test at command line or adb with:
am broadcast -a android.provider.Telephony.SECRET_CODE -d android_secret_code://XXXX where xxxx is code