NEC Univerge sv8500 Manual page 1281

For a tandem connection involving Central Office Trunk (COT) and Loop Dialing Trunk (LDT), input level
Note 2:
can be lower than minimum DTMF detection level (-27.0 dBm) at MG(PRI) because of Line Loss between
two switches.
If input level at MG(PRI) is lower than minimum DTMF detection level,
DTMF relay cannot be guaranteed.
STN A
ISDN
DTMF
relay
Input
MG
VOL.2-1028
MG(PRI) (1.5M) [SCA-24PRIA/MG-24PRIA]
2-wire
Line Loss
The signal can be attenuated here.
If input level is smaller than -27 dBm,
DTMF relay cannot be guaranteed.
COT/LDT
LC/LDT
LC
STN B