The USSD protocol allows you to access hidden features you didn't know about right from your smartphone's dialer. But there is some trickiness you'll need to know about.

Coders have a storied tradition of baking in secret passageways that can only be accessed by inputting a special "key." And so that tradition continues in the mobile age.

These publicly available backchannels allow users to directly communicate with their service provider's computers and/or access back-end features in their device. They are accessed by inputting them into the phone's dialer (the screen you use to start a phone call) and usually begin and end with the * or # keys with a sequence of numbers in between (there's close-to-zero chance that anyone would accidentally access them).

Most people don't really need to know how their local cell towers are performing or what their IMEI number is (more on that later). Still, it can be fun to play around and see what unexpected functionality your phone is hiding beneath the surface.

If you really want to try them out, your best bet may be to Google your phone's make and carrier + "USSD" for a tailored, comprehensive list.

Field Mode: *3001#12345#*
Type *3001#12345#* into your phone's dialer and then press the green call button to access "Field Mode," which can give you access to info about local networks and cell towers.

You'll probably never ever have to know about your local cell tower's "Measured RSSi," but it's fun to look around for a bit.
General Test Mode: *#0*#

It works on Android. This prompts a library of different phone operations, which could be operated with a single push (e.g. Sleep, Front Cam, Vibration).

Display your IMEI: *#06#
To access it, type in the above code, and then the green call button to prompt your IMEI number.
The IMEI is unique to your device. Among other things, the number can help "blacklist" stolen devices or help with customer support.
Check Your Call Forwarding: *#67#
This code allows you to check which number your phone is currently forwarding calls to when you're busy or reject a call.

By default, this is probably your carrier's voicemail service, but you can change it to forward to a different number (a home number, office number, or third-party answering service for example). On an iPhone, you can change this number by going to Settings > Phone > Call Forwarding. On Android (varies from system to system), tap the Phone app > hamburger icon > Settings > Call > More Settings > Call forwarding
Get Even More Info on Call Forwarding: *#61#
On Galaxy phone, this code prompted a pop-up that explain how long until a call is forwarded to the message center. On the iPhone, regardless of carrier, this code just show the same info as *#67# .

Check Your Available Minutes: *646#
Apparently this one only works on postpaid plans. It work on Galaxy phone. Instead of showing the info on a new screen, it send to the phone a text message.
Check Your Bill Balance: *225#
On Android it prompt a SMS message with current balance due.
Hide Your Phone From Caller ID: #31#
Works on Android. But entering this code prompted a pop-up stating that Caller ID had been disabled. In order to re-instate Caller ID, enter *31# .

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Check Your Billing Cycle: *3282#
Works on Android. It prompt an SMS message with billing info.
SMS Message Center: *5005*7672#
This code will tell you your SMS message center number.
Activate Call Waiting: *43#
This code will activate call waiting; you can deactivate it by entering #43#.
Quick Test Menu (Samsung Galaxy Only) *#7353#
This code only works on Samsung Galaxy models. This is similar to the General Test mode mentioned earlier, in that it brings up a menu with a number of one-tap test prompts.
 
Firmware (Samsung Galaxy Only) *#1234#
Only works on Galaxy devices. But it will let you know your phone's current firmware. So, have fun with that.

Codes working with most of cell phones
These code should work with most cell phones. Some may not be usable depending on the provider.