We have users who wish to have a common phonebook available on multiple 7925 phone sets
What about using 3rd party solution?
E.g. 2Ring PHONE SERVICES can resolve your request: http://www.2ring.com/NewJuice.
I've used a personal address book for this in the past and just had it logged in with that user account on all the devices. Probably easier/better to just use a phone service for this instead though.