Справочник Пользователя для Netopia PN Series

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.

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.