Welltech WG2504 FXS V.4.02 - Manual (Page 82)

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Figure 19. Disconnect tone table.
WellGate 2540 supports an alternative feature to release FXO port by
detection Loop Current Drop signal. Go to FXO Setting  Select FXO Line 
Modify Line Setting  Current Drop for Disconnect to
YES to enable this
feature. Once calling user drop this call, PSTN or PABX may drop Loop Current
signal for about 300 msec (default setting) to inform FXO interface. To detect
Loop current drop duration can be changed. Go to FXS Setting  Others 
Current Drop Time . See Figure 21. This feature depends on the PSTN switch
or PABX you are using. See Figure 20 at webpage configuration.
Figure 20. Loop Current Drop to release FXO port