I have the V551 and have asked the same question.  From what I have
read in the newsgroups its inherent to the phone, not the headset that
custom ringtones are disabled.  However, if you want to get into
hacking the phone, there is a way to change the ringtone that does
play, to whatever ringtone you want.  You still will not have specific
ringtones based on who is calling you but you can change the default
ringtone for use with a BT headset.

I've never done it and would have to research it more if I wanted to.
It involves editing a "seem" files and uploading them to the phone.
If you want to pursue it, go to www.howardforums.com and search around
there.  That's where I'm sure I will end up the day I decide to modify
mine.  Hope that helps.

H