📱 Complete Guide to GSM Voice Services & USSD Codes
Master Call Management, Divert Services & Network Commands
📋 Table of Contents
1. Introduction to GSM & USSD
📡 What is GSM?
GSM (Global System for Mobile Communications) is a digital mobile network standard that is widely used by mobile phone users around the world. It serves as the foundation for 2G, 3G, and 4G networks, enabling voice calls, text messaging, and data services.
📢 What is USSD?
USSD (Unstructured Supplementary Service Data) is a communication protocol used by GSM cellular telephones to communicate with service provider computers. USSD codes are typically used to access hidden features and network services by dialing specific code sequences.
USSD is session-based, meaning that when a user accesses a USSD service, a session is established, allowing for real-time communication between the mobile phone and the network. Unlike SMS, USSD messages are not stored; they are transmitted in real-time during the session.
2. Understanding USSD Code Structure
USSD codes follow a specific structure that consists of various components. Understanding this structure is essential for effectively using these codes:
Code Components Explained:
- ** (Double Star): Indicates the start of a registration or activation command
- ## (Double Hash): Used for deactivation or cancellation commands
- *# (Star Hash): Used to check the status of a service
- Registration Code: Specifies the type of service (e.g., 21 for call divert)
- Service Code: Optional parameter for specific service conditions
- Phone Number: The destination number for call forwarding services
- # (Hash): Indicates the end of the command
3. Call Divert Configuration
Call divert (also known as call forwarding) allows you to redirect incoming calls to another number. This is particularly useful when you're unavailable, out of coverage, or want all calls directed to a specific number.
3.1 Unconditional Call Divert (Code 21)
| Action | USSD Code | Description |
|---|---|---|
| Activate | **21*PhoneNumber# | Diverts all calls to specified number |
| Deactivate | ##21# | Cancels unconditional call divert |
| Check Status | *#21# | Shows current divert status |
3.2 Divert When Unanswered (Code 61)
| Action | USSD Code | Description |
|---|---|---|
| Activate | **61*PhoneNumber# | Diverts unanswered calls |
| Activate with Timer | **61*PhoneNumber**Seconds# | Set delay (5, 10, 15, 20, 25, 30 seconds) |
| Deactivate | ##61# | Cancels divert when unanswered |
| Check Status | *#61# | Shows current settings |
3.3 Divert When Unreachable (Code 62)
| Action | USSD Code | Description |
|---|---|---|
| Activate | **62*PhoneNumber# | Diverts when unreachable |
| Deactivate | ##62# | Cancels unreachable divert |
| Check Status | *#62# | Shows current settings |
3.4 Divert When Busy (Code 67)
| Action | USSD Code | Description |
|---|---|---|
| Activate | **67*PhoneNumber# | Diverts when busy |
| Deactivate | ##67# | Cancels busy divert |
| Check Status | *#67# | Shows current settings |
3.5 Combined Divert Codes
| Code | USSD Command | Description |
|---|---|---|
| 004 | **004*PhoneNumber# | Activates codes 61, 62, and 67 simultaneously |
| 002 | **002*PhoneNumber# | Activates ALL divert codes (21, 61, 62, 67) |
| Deactivate All | ##002# | Deactivates all call divert services |
4. Call Waiting Services
Call waiting allows you to receive notifications of incoming calls while you're already on a call. This service enables you to put the current call on hold and answer the new incoming call.
| Action | USSD Code | Description |
|---|---|---|
| Activate | **43# | Enables call waiting |
| Deactivate | ##43# | Disables call waiting |
| Check Status | *#43# | Shows call waiting status |
5. Voice SMS Services
Voice SMS (also known as Voice Mail) allows callers to leave voice messages when you're unable to answer. These messages are stored on the network and can be retrieved later.
| Service | USSD Code | Description |
|---|---|---|
| Access Voicemail | *#111# | Retrieve voice messages |
| Alternative Access | Call 121 | Direct voicemail access number |
6. Complete USSD Codes List
Here's a comprehensive list of the most commonly used USSD codes across GSM networks:
6.1 Call Management Codes
| USSD Code | Function | Category |
|---|---|---|
| *#06# | Display IMEI number | Device Info |
| *#21# | Check call forwarding status | Call Divert |
| **21*Number# | Forward all calls | Call Divert |
| ##21# | Cancel all call forwarding | Call Divert |
| *#30# | Check calling line presentation | Call Settings |
| *#31# | Block caller ID | Privacy |
| #31#Number | Hide caller ID for one call | Privacy |
| *31#Number | Show caller ID for one call | Privacy |
| *#43# | Check call waiting status | Call Waiting |
| **43# | Activate call waiting | Call Waiting |
| ##43# | Deactivate call waiting | Call Waiting |
| *#61# | Check number for unanswered calls | Call Divert |
| *#62# | Check number for unreachable calls | Call Divert |
| *#67# | Check number for busy calls | Call Divert |
| ##002# | Cancel all call diverts | Call Divert |
6.2 Network & Account Information
| USSD Code | Function | Note |
|---|---|---|
| *#100# | Check account balance | Carrier-specific |
| *#123# | Check balance & services | Carrier-specific |
| *#121# | Access voicemail | Common code |
| *#225# | Check bill balance | Postpaid accounts |
| *#62209526# | Display network IP & MAC | Technical info |
6.3 Service Control Codes
| USSD Code | Function | Description |
|---|---|---|
| *#33# | Check call barring status | View outgoing call restrictions |
| *#35# | Check incoming call barring | View incoming call restrictions |
| *#76# | Check connected line presentation | Network feature |
| *#77# | Check connected line restriction | Network feature |
7. Important Notes & Tips
Key Points to Remember:
- Network Storage: All voice service configurations are saved on the network operator's server, ensuring portability across devices
- Carrier Variations: Some USSD codes may vary depending on your mobile carrier. Check with your provider for specific codes
- International Format: When entering phone numbers for call divert, use the international format (e.g., +1234567890)
- Timer Settings: For code 61, you can set delays between 5 to 30 seconds in 5-second increments
- Billing Awareness: Call forwarding services may incur additional charges - review your plan details
- Testing: After setting up call divert, test it by calling your number from another phone to ensure it works correctly
- Deactivation: Remember to deactivate services when no longer needed using the ## codes
- Multiple Services: You can have multiple call divert types active simultaneously (e.g., 61, 62, and 67)
Troubleshooting Tips:
🔧 Common Issues & Solutions
- Code doesn't work: Ensure you're entering the exact code format with all asterisks and hashes
- Service not activating: Check if your carrier supports the specific service
- Unexpected charges: Review your plan's call forwarding rates before activation
- Can't deactivate: Use ##002# to cancel all call diversions at once
Conclusion
Understanding and utilizing USSD codes for GSM voice services gives you complete control over how you manage incoming calls. Whether you need to forward calls when you're busy, unreachable, or simply want all calls redirected to another number, these powerful codes provide the flexibility you need.
By mastering these codes, you can enhance your mobile communication experience, ensure you never miss important calls, and manage your availability more effectively. Remember to always check your carrier's specific implementation and billing policies for these services.
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