D-4 Netopia ISDN Router Reference GuideBackup numberAnother use for the Optional Second Number item is for storing abackup number in case a connection cannot be made using theprimary number. For example, if calling the primary number returnsa busy signal, the Netopia ISDN Router will attempt to use thesecondary number stored in Optional Second Number. If aconnection cannot be made using the secondary number, theNetopia ISDN Router will retry the primary number.If the remote network's directory number is part of a hunt group, asecond directory number may not be necessary to make asuccessful connection using two B-channels.Dynamic B-channel usageIf the B-Channel Usage item in a connection profile’s PPP/MPOptions screen is set to Dynamic or 2 B, Pre-emptible, one or bothB channels may be in use at any time during a call made with thatconnection profile. Use of the second B-channel depends on trafficvolume.In addition, one of the B-channels may be relinquished if there is anincoming call, or if a second outgoing connection is made usinganother connection profile.The ability to allow incoming calls when both B-channels are in usedepends on the type of switch protocol on the local ISDN line, andhow that line is provisioned (configured). Some types of switchprotocols never allow incoming calls when both B-channels are inuse. Switch protocols that do allow incoming calls must have theadditional call offering (ACO) parameter turned on for data. ACO fordata is off by default.
