Thanks Jinnie, from me also.
This worked on my HTV One V with 4.0.3, just a couple of changes -
Tap Settings,
Call,
Call Forwarding,
Forward when Unanswered,
Tap Delay (in my case it was 5 seconds (!!)), select your delay time, tap Update.
So now my 30 second ringtone from the Rolling Stones plays in full. Bonus !
(I used Audacity to 'hand craft' a seamless loop to make my ringtone.)