I spent a week punching in obscure character strings on both an iPhone and a stack of Android phones, and it turned my dialer into a control room. These “MMI/USSD” codes—baked into GSM/LTE standards and selectively enabled by carriers—surface hidden menus, network diagnostics, and service toggles that you won’t find on a home screen.
Most are safe; a few can make major changes. Always double‑check before you dial, and back up your data first. On iPhone, nearly all codes require tapping the call button. On Android, codes with *#*#…#*#* typically execute automatically as soon as you type the final star-pound sequence.
iPhone codes: field test and core network checks
1) *3001#12345#* — Field Test Mode: This opens Apple’s hidden radio diagnostics panel, exposing signal strength in dBm (lower is weaker; around −70 dBm is strong, −110 dBm is poor), LTE cell metrics, and neighboring cells. It’s invaluable for figuring out dead zones indoors. Apple’s support materials and 3GPP specs describe these measurements, but the menu is the fastest way to see them live.
2) *#06# — IMEI/EID display: Instantly shows your IMEI (and EID for eSIM). Carriers and law enforcement use the IMEI to blacklist stolen devices via the GSMA IMEI database, and the FCC recommends having it on file for theft reports and warranty claims.
3) *#21# — Call forwarding status (all): Returns whether unconditional forwarding is enabled. If your iPhone sends every call to voicemail without obvious reason, this check solves the mystery in seconds.
4) *#61# — Forward when not answered: Reports the current target number and ring timeout for “no answer” forwarding, useful if you want missed calls to roll to a secondary line after a set number of rings.
5) *#62# — Forward when unreachable: Shows where calls go when your phone has no signal or is powered off—handy when traveling or relying on spotty building coverage.
6) *#67# — Forward when busy: Confirms the destination if you’re on another call. It’s a quick way to verify that call overflow routes to a colleague or a second device.
7) *#43# — Call Waiting status: Tells you if Call Waiting is active. Many networks still honor *43# to enable and #43# to disable, but iPhone Settings > Phone remains the reliable toggle if your carrier blocks MMI changes.
8) *#31# — Caller ID restriction status (CLIR): Returns whether your number is hidden by default. For one‑off overrides, most US carriers accept *82 before the number to show your ID on the next call.
9) *5005*25371# — Test Alerts status: Confirms whether FEMA/WEA test alerts are allowed on the device. If you need to toggle test participation without digging into Settings, some networks accept *5005*25370# to change it. This affects test alerts only; public safety alerts remain controlled by carrier and system policy.
Android codes: diagnostics and hidden panels
10) *#*#4636#*#* — “Testing” menu: The Swiss Army knife. You’ll see Phone information (radio tech, data network type, signal in dBm and ASU), usage stats, and details for Wi‑Fi and SIMs. Engineers use it to verify VoLTE/NR registration and observe cell handoffs. The Android Open Source Project documents much of what you’ll see here.
11) *#*#426#*#* — Google Play services diagnostics: Opens a status screen for FCM (push notifications), including connection state and heartbeat. If your messaging apps lag, this is a fast sanity check.
12) *#*#225#*#* — Calendar storage view: Surfaces a list of upcoming events pulled from Calendar Provider. It’s a niche tool, but great for confirming whether sync is actually populating the device database.
13) *#0*# — Samsung hardware test: On many Galaxy models, this diagnostic hub checks display color patterns, touch layers, sensors, camera modules, and vibration motors. It’s the same screen many repair centers use before and after a fix.
14) *#*#7780#*#* — Factory data reset (danger): Immediately prepares a wipe of apps and data on many Android devices. Do not dial this unless you’re intentionally resetting. Back up first, confirm your cloud restores, and mind that this is irreversible once triggered.
Carrier caveats and why some codes fail
Results vary by network. Operators can disable or reinterpret MMI codes at the switch, especially on VoLTE and 5G cores. MVNOs may support fewer commands than the host network. If a code returns “Error” or does nothing, it’s usually a carrier policy choice, not your phone.
Legacy differences linger, too. Many codes came from GSM standards; devices migrating from older CDMA networks sometimes ignore them. CTIA and carrier support pages note that forwarding and caller‑ID controls increasingly live in app settings rather than the dialer.
Practical uses and pro tips
Use Field Test (iPhone) or Testing (Android) to capture repeatable signal readings when choosing a home office spot. In my own checks, simply moving a desk one room over improved LTE RSRP from roughly −108 dBm to −89 dBm—enough to stabilize calls. Pair this with a speed test for a fuller picture.
Keep your IMEI handy for theft or warranty issues; consumer advocacy groups and the FCC recommend recording it the day you activate a device. And if you rely on conditional forwarding for business, verify *#61#, *#62#, and *#67# after swapping SIMs or carriers—those settings don’t always migrate cleanly.
If a code doesn’t work
Try again with Wi‑Fi Calling off, confirm you’re using the active SIM line, and reboot the phone. If you’re on an MVNO, check its support pages for supported short codes. When all else fails, use the official paths: Apple’s Settings > Phone, Android’s Phone app settings, or your carrier account app.
One last reminder: treat any reset or provisioning code with caution. A minute of prep—backups, sign‑in checks, and reading the on‑screen prompt—beats hours of recovery every time.