KX-TG6561CAT, KX-TGA651RUT - Panasonic Telephone Service Manual (repair manual). Page 13

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In order to disable call waiting and stutter tone functions when using telephones connected in parallel, it is necessary to have a
circuit that judges whether a telephone connected in parallel is in use or not. This circuit determines whether the telephone
connected in parallel is on hook or off hook by detecting changes in the T/R voltage.

Parallel connection detection when on hook:
When on hook, the voltage is monitored at pin 17 of IC501. There is no parallel connection if the voltage is 
0.54 V or higher, while a parallel connection is deemed to exist if the voltage is lower.

Parallel connection detection when off hook:
When off hook, the voltage is monitored at pin 18 of IC501; the presence/absence of a parallel connection is determined by
detecting the voltage changes.
If the Auto disconnect function is ON and statuses are Hold, receiving ICM, OGM transmitting, BBIC disconnects the line after
detecting parallel connection is off hook.

Q141

Q161

R141

R142

Q142

R117

R115

R116

19 pin

IC501 DCIN 3

IC501 DCIN 1 17 pin

R118

R151

R145

Pin 18 of IC501 

DCIN2

HOOK 

Pin 6 of IC501

C101

D101

C102

SA101

P101

TEL  JACK

3

4

2

1

R163

R167

L1T

L1R

R152

C152

RA151