Apple Remote Access MultiPort Server - Manual (Page 23)

Dial-in Access allows dial-in and callback access on the same port. With this
method, a user who does not have callback enabled is able to dial in and
remain connected. This is useful if you have a number of users who are on the
road and cannot provide you with a callback number. If the user has callback
enabled, when the user dials in, the server checks that the user name and
password are correct. If they are correct, the user is disconnected, and the
server calls him or her back on the same port.
Ports set to Dial-in and Transfer are intended only for users who have callback
enabled. When such a user dials in, the server checks that the user name and
password are correct. If they are correct, the user is disconnected, and the
server calls him or her back through a different, callback-only port. Users for
whom you have not set up callback can dial in to Dial-in and Transfer ports,
but they will be disconnected. Users who do not have callback enabled must
use Dial-in Access ports.
Callback Only ports cannot receive calls. They are used only in conjunction
with ports that have been configured for Dial-in and Transfer. For example, you
can set up one port for Dial-in and Transfer and the rest for Callback Only.
Thus, you can have a maximum of 15 callback lines, depending on the number
of expansion cards installed and modems or other serial communications
devices connected to the ports. This method allows you to provide only one
telephone number to your remote access users.
Note: If you configure one port as Dial-in and Transfer and the rest as Callback
Only, the user will see the following alert box when all callback lines are in use:
Configuring the modem ports 23