These test results are probably unique to my Panasonic home phone system so your results will probably be different but useful.
My handsets have both Hold and Flash buttons.
In these tests the calling phone is a conventional land line, the called phone is connected to the ATA.
Both phones are on my two-line home phone system so the handsets are identical.
The Grandstream method for putting a call on hold is to press the flash button if the analog phone has one… Doing that on the called phone hangs up the call.
Pressing the Hold button on the called phone puts the call on hold BUT while on hold I hear a LOUD dial tone on the called phone, silence on the calling phone.
Pressing the Hold button on the calling phone puts the call on hold but while on hold I hear a QUIET dial tone on the calling phone, silence on the called phone.