USSD codes for voice services on your GSM Sim networks

GSM or the Global System for Mobile Communications, is a standard developed by the European Telecommunications Standards Institute to describe the protocols for second-generation digital cellular networks used by mobile devices even though now it supports up-to 4G LTE protocols . As of 2014, it had became the global standard for mobile communications – with over 90% market share, operating in over 193 countries and territories.
On the other hand , USSD codes or the Unstructured Supplementary Service Data (USSD), sometimes referred to as "Quick Codes" or "Feature codes", is a communications protocol used by GSM cellular telephones to communicate with the mobile network operator's computers. USSD can be used for WAP browsing, prepaid callback service, mobile-money services, location-based content services, menu-based information services, and as part of configuring the phone on the network.

USSD messages are up to 182 alphanumeric characters long. Unlike Short Message Service (SMS) messages, USSD messages create a real-time connection during a USSD session. The connection remains open, allowing a two-way exchange of a sequence of data. This makes USSD more responsive than services that use SMS.

Now , further understanding of a few of the common USSD codes is as follows,USSD codes works in the same way on all networks and GSM phone. USSD codes always looks like:

• Activate: *USSD_Code# @

• Deactivate: #USSD_Code # @

• Delete: ##USSD_Code # @

• Check Status: *#USSD_Code# @

USSD_Code = the feature code the mobile phone should send to the network
@ = Send (call button)

Configuring Call Diverts And Personalized  Voicemail Services using USSD codes

Divert all Calls (21)

All your incoming calls will be forwarded, doesn't matter if your phone is switched on or not. The USSD
code is "21".

• Forwarding: **21*DESTINATION_NUMBER# @

• Activate: *21# @

• Deactivate: #21# @

• Delete: ##21# @

• Check Status: *#21# @

Divert if no Reply (61)

All the incoming calls will be forwarded if not taken within a defined time. The USSD code is "61".

• Forwarding: **61*DESTINATION_NUMBER# @

• Activate: *61# @

• Deactivate: #61# @

• Delete: ##61# @

• Check Status: *#61# @

In addition you can tell the network after which time the call should be diverted. Here the GSM specifications
define a time between 5 to 30 seconds:

Forwarding: **61*DESTINATION_NUMBER*DELAY# @

The DELAY time can be 5, 10, 15, 20, 25 or 30 seconds.


Divert if Unavailable (62)

All the incoming calls will be forwarded if the phone is switched off or out of network coverage. The USSD
code is "62".

• Forwarding: **62*DESTINATION_NUMBER# @

• Activate: *62# @

• Deactivate: #62# @

• Delete: ##62# @

• Check Status: *#62# @

Divert if Busy (67)

All the incoming calls will be forwarded if another call is in progress. The USSD code is "67".

• Forwarding: **67*DESTINATION_NUMBER# @

• Activate: *67# @

• Deactivate: #67# @

• Delete: ##67# @

• Check Status: *#67# @

All Conditional Diverts (004)

You can also activate, switch on/off, check or delete all conditional forwards (61, 62 and 67) with just
one USSD code. The USSD code is "004".

• Forwarding: **004*DESTINATION_NUMBER# @

• Activate: *004# @

• Deactivate: #004# @

• Delete: ##004# @

• Check Status: *#004# @

Deactivate all Diverts (002)

This USSD code deactivates all call forwardings with one single command. The USSD code is "002"

##002# @

Please note that using the unconditional "Divert all Calls (21)" overwrites all conditional call forwarding!

Call waiting (43)

Call waiting provides you with notification of an incoming call while on another call and allows you to to handle call or take a call that is waiting.

• Activate: dial *43#

• Deactivate: dial #43#

• check status: dial *#43#

Voice SMS (1)

 You can make a voice sms at the rate of ksh 3.50 safaricom(kenya) rate by dialing 1 followed by the number e.g 10722004000
N/B. This works only on safaricom numbers, because its a customized service for safaricom customers only. 

In conclusion, i would say that these features were available since the first GSM networks were launched and the phone menu doesn't do anything different than sending these codes. However, this gives you a faster access to your settings and in addition it offers you one feature which isn't available through most mobile phone menus (defining after which time a call should be forwarded if you don't reply). Please remember that this information is stored on the GSM network, not on your GSM device. So doesn't matter if you change your SIM card to another terminal, the settings are stored as long as you don't change it.

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