Did you read the configuration on how to enable call forwarding for youmail?
I got it to work on my inspire.
Step #1) Dial the following number (including the * as if you were making a call) to have YouMail receive your voice mails:
*004*17142043114#
Step #2)
Once you receive confirmation you have set call forwarding, press the END button.
(Your phone will display "Forward Set: All Calls" or something similar)