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TANDEM TRUNKING (V2)
Description
Tandem trunking provides an enhanced networking capability for System 25. With this
feature, tie trunks can be used to call through System 25 to reach another switching system
(CO or PBX ). Calls may be completed over on-network or off-network facilities.
To be treated as tandem trunks, tie trunks must be assigned trunk numbers beginning with
9. Incoming calls on these trunks may route out of System 25 over ground start, loop start,
or tie trunks. Tandem trunks can gain access to outgoing facilities either indirectly (by the
ARS feature) or directly (by dial access).
System 25’s Tandem Trunking feature does not support traveling class marks or centralized
attendant service.
Considerations
The use of tandem trunking with tie trunks provides a cost-effective alternative to toll
calling between branches.
Interactions
● Automatic Route Selection: Tandem trunk calls that route outbound via ARS
receive the same treatment as calls originated by a System 25 station, with one
exception. If all facilities in a routing pattern are busy, call queuing is not provided.
In this case, busy tone is returned to the calling party.
The second digit of the trunk number is used to specify the trunk’s “ station” Facility
Restriction Level (FRL) for use with ARS. FRLs may be specified as follows:
Trunk Number
I
Second
I
Station
Range Digit FRL
9000-9199
0orl
0
9200-9399
2or3
1
9400-9599
4or5 2
9600-9999
6, 7, 8,
3
or 9
To gain access to an ARS routing facility, the tandem trunk’s “station” FRL must be
equal to or greater than the route’s FRL. Thus, a tandem trunk with an FRL of 0
has the least ARS privileges, while an FRL of 3 provides the most privileges. If the
restriction level of the tandem trunk is less than all route FRLs, reorder tone is
returned to the calling party.
● Dial Access: No toll restriction is provided for tandem trunk calls. However, access
to outgoing facilities can be controlled via the “allow dial access” option in the
outgoing trunk’s administration. Tandem Trunk calls receive the following treatment
when attempting dial access of System 25 facilities:
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