Netopia Router PN Series - Manual (Page 256)

C-4 Reference Guide
Backup number
Another use for the Optional Second Number item is for storing a
backup number in case a connection cannot be made using the
primary number. For example, if calling the primary number returns
a busy signal, the Netopia Router will attempt to use the secondary
number stored in Optional Second Number. If a connection also
cannot be made using the secondary number, see the event history
to determine the problem and its solution. See Appendix E, “Leased
line and ISDN Events”, for more information.
If the remote network's directory number is part of a hunt group, a
second directory number may not be necessary to make a
successful connection using two B-channels.
Dynamic B-channel usage
If the B-Channel Usage item in a connection profile’s PPP/MP
Options screen is set to Dynamic or 2 B, Pre-emptible, one or both
B channels may be in use at any time during a call made with that
connection profile. Use of the second B-channel depends on traffic
volume.
In addition, one of the B-channels may be relinquished if there is an
incoming call, or if a second outgoing connection is made using
another connection profile.
The ability to allow incoming calls when both B-channels are in use
depends on the type of switch protocol on the local ISDN line, and
how that line is provisioned (configured). Some types of switch
protocols never allow incoming calls when both B-channels are in
use. Switch protocols that do allow incoming calls must have the
additional call offering (ACO) parameter turned on for data. ACO for
data is off by default.