Definition: the call forward is a phone feature, which allow users to forward their VoIP incoming calls to mobile or landline numbers (it is possible also between mobile numbers).
This feature can be activated in different scenarios: when the line is busy, if there is no answer or permanent call forwarding (all incoming calls are forwarded). 
A detailed description can be found on wikipedia.

How to setup call forward?

The call forward can be configured at server side and also at client side.

On the Mizu softswitch it can be done at Users and devices form/ Functions page
and in the Mizu universal browse phone there are the following parameters for this purposes: callforwardonbusy, callforwardonnoanswer, callforwardalways

- callforwardonbusy: the calls are forwarded to the specified phone number, when the called party is already in another call

- callforwardonnoanswer: the calls are forwarded to the specified phone number, when the called party is not answering

- callforwardalways: all calls are forwarded to the specified phone number


Benefits of the call forwarding:
- availability: the calls to a softphone or ATA can be easily forwarded to the users mobile number or home landline phone, thus he will be available
even when he is out of office or he hasn't internet connection.
- avoiding voicemail: the user can avoid the usage of voicemail
- increasing presence: even if some employees are out of their offices, their calls can be handled by their colleagues

Disadvantages of call forwarding:
- forgetting to turn off: in many cases the call forwarding is required just for a specified time interval (for example:weekends, holidays, during the nights) to be sure that there are not missed calls. Forgetting to turn off this feature, can be a problem if somehow the cellphone is not around or its battery is empty.

Below you can find a call flow about call forwarding when called party is busy: