AT&T AT&T manual Tandem Trunking, S c r i p t i o n, Trunk Number C o n d A t i o n, 236

TANDEM TRUNKING (V2)

D e s c r i p t i o n

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	S e c o n d	I	S t a t i o n
R a n g e	D i g i t		F R L
9000-9199	0 o r l		0
9200-9399	2 o r 3		1
9400-9599	4 o r 5		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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