Other Incoming Call Restrictions - Netopia Router PN Series Reference Manual

The table below shows which supported switch types can allow an
incoming call when ACO for data is on.
Type of switch:
AT&T 5ESS custom
AT&T 5ESS National ISDN-1
DMS-100 custom
DMS-100 National ISDN-1
* Must have two directory numbers assigned.
This table may not be a complete list of switch protocols that
support ACO. To find out if your switch protocol supports ACO, or to
turn ACO on, contact your ISDN service provider.

Other incoming call restrictions

A B-channel will not be relinquished to admit an incoming call if a
connection profile has B-Channel Usage set to 2 B-Channels.
A B-channel will not be relinquished to admit an incoming call when
there are two separate concurrent calls. Incoming calls are
automatically allowed in when there is at least one B-channel free.
ISDN Configuration Guide
Incoming call allowed?
N/A
Yes
Yes*
Yes*
C-5