Typically your modem call arrives on some known CLI/DID/whatever for which an Incoming Call Route is set to RAS service as its destination.
RAS username/password handshake occurs and then the connection is assigned an IP address by the IP Office (by default).
That connection is then the same as a PC on the same LAN as the IP Office, ie. run Manager, the only change is that you may have to change the Manager preferences from 255.255.255.255 to the specific address of the IP Office LAN1.