You can program this onto either one of the line buttons or one of the programmable buttons on your huawei phone. To do so press and hold the line button or programmable button you wish to setup the unconditional forward to voicemail on.
Once you have done the long press you will be given the below screen. Press 6 on the numpad below the screen to select Call Forward.
Press the softkey Select while All to is highlighted.
Press the softkey Select Voicemail is highlighted to forward all calls to voicemail.
You will now be taken back to the homepage of your phone. To turn on or turn off the unconditional call forwarding simply press the same button that you selected at the start of this. When you turn it on you will see the below message appear.
The corresponding button will also have a lit indicator to show that it is active.