I encountered this issue and luckily I was able to find a blog entry about it:
Essentially Cisco phones use CDP to discover their voice vlan. HP procurve phone’s use LLDP to accomplish the same goals. Now apparently this works fine with the Cisco 79XX model phones which are there Enterprise class phones.
However I was trying it with the Small Business line 500 series phones and it does not work. According to the article you need to manually disable CDP on the phones to make this work. The other alternative (which is what I did) is manually input the voice VLAN into the phone.
Hope this helps