You need to mention this in your PSTn Truk Group config sample as below
PSTN Trunk Group
—————-
PSTN Trunk Group: 1
Name = PstnPassThrough1
Pass Through = no(0)
PT Trunk ID = 0
Provide Call Progress Tone = no(0)
Busyout = no(0)
Hunt Algorithm = ascending(0)
Modem Calls = No(0)
Direction = incoming(0)
DN Used = public
End Of Dial = yes(1)
End Of Dial Digit = #