HTC One M9

Find "Call forwarding"

Slide your finger down the display starting from the top edge of your phone.
Press the settings icon.

Find "Call forwarding"

Press Call.

Find "Call forwarding"

Press Call forwarding.

Select divert type

The current settings are displayed.

Select divert type

Press one of the following options:
Always forward

Select divert type

Forward when busy

Select divert type

Forward when unanswered

Select divert type

Forward when unreachable

Divert calls to voicemail

Key in (prefix) 5 (your phone number)
If you selected Forward when unanswered in step 2, you need to select a delay for the divert:

Divert calls to voicemail

Press the drop down list below "DELAY".

Divert calls to voicemail

Press the required delay.

Divert calls to voicemail

Press Enable.

Return to the home screen

Press the home icon to return to the home screen.