Call Forwarding or Call Diverting is explained.

Call forwarding means, is to divert a call coming from your phone to another number.  This is also called call diverting, adding more revenue to the mobile operator, and providing a very important feature to the mobile users. If the call forwarding is enabled for a mobile subscriber, then he can receive the calls on another number. 

For example, if a mobile user moves to another country, and the calls in roaming are very costly.  But at the same time, he does not want to miss the calls on the original number.  He buys a new sim card, local to the roaming country, and enables call forwarding to a new number from the home sim. 

Call forwarding provides more revenue to mobile network operators, now there are two calls in place of one.  While doing call forwarding, there are two call legs, one from the original mobile to the called party and another form called party to the forwarded number.

Call forwarding and Call transfer:

In both, the call is sent to a new number other than the originally dialed number.  In Incall transfer, the original call is put on hold. the new number is dialed and once the call on a new number is picked, the two calls are connected.  The call transfer does not need any settings in the profile of the mobile subscriber on HLR.

In the Incall transfer, there is only one leg for the call. So this leads to no additional revenue for the mobile operator.  You might experience call transfer when called in some company office. In the company office at reception, the guy picks up the call and connects you to the desired person. And the reception guy gets disconnected from the call.

Call forwarding types:

A mobile subscriber has flexibility for call forwarding. The feature may be active in different scenarios.  During roaming, a mobile subscriber may lose the signal. So he wants to set the call forwarding to his landline number if the phone is not reachable.  Following is the list of call forwarding types.

Call forwarding unconditional : 

As the name suggests, if the call forwarding or call diverting for unconditional is set, then all incoming calls will be forwarded to the other number.  Even if a mobile subscriber switched off the phone,  the calls will be forwarded. Unconditional call forwarding is useful when you know that the original phone will not use at all. Like in case, you have purchased a new number and want to update a new one to all of your contacts. But meanwhile don’t want to lose the calls coming on the older phone number.

Unconditional call forwarding call flow in GSM:

Call forwarding conditional: 

When conditional call forwarding is enabled.  Call diverting depends on subscriber status.  Following is the conditional call diversion scenario.

Call forwarding busy:

A mobile user talking to another person.  In this the status of the user is busy.  At a single time, the user can receive only one call.  If call forwarding busy is not active, the new call will disconnect.

call forwarding not reachable:

In situations where mobile phones get disconnected from the network. Call forwarding is not reachable is set. Then the forwarded number will start ringing.

call forwarding no answer:

For an incoming call, where the phone keeps ringing for some time, and the call is not picked up. Now, this call forwarding comes into the picture.  If a mobile subscriber has activated this call forwarding, then after ringing for some time call forwards to the forwarding number.

Call forwarding data on HLR:

Similar to any other service, the subscriber profile on HLR has a call forwarding subscription for each call forwarding type. With the subscriber provisioning, call forwarding is in the registered state. Later a subscriber can enable or disable each call forwarding type. The following are the parameters in the subscriber profile.  All types are described in GSM 03:08.

  1. Call forwarding type.
  2. Call forwarding state.
  3. The number for forwarding.