These settings can only be done by a field engineer. All devices have a technician mode. It is in this mode that this kind of changes can be done. Please consider to call your local support center.
About the settings, the fax from this devices don't support VoIP and even with an adapter you may experience problems. The settings recommended by IP phone seams to be OK, but some doens't exist. The most important is baud rate of 9600. In same cases you need to decreases even more. You may experience increase of time to send a fax, but it is because of this rate.
Any way, Xerox has an adapter to these situations. If the one provided by IP phone doesn't work you my want to try the Xerox one.
I have a Xerox WorkCenter 7435. We recently got an IP phone system. We received an adapter for the faxes, however the 7435 will not work. The IP phone tech support is telling me to make sure the following items are set.