if you look at the sheet for the 7905G this is how it works, my only thought is that these phones are really to be used in areas like receptions etc, so the need for hands free functionality is not really needed. 7910's are also like this.
on the 7905 and 7910 they do not have speaker phone only on hook dialling
your cisco provider should have really told you this, if this is a functionality that you wanted.
hope this helps
dave