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Polarity Reversal - The first step will be to try enabling Polarity Reversal option.
In the WebGUI please navigate to: Expert Config > POTS > FXO Port Hardware Profiles > Line Reversal - mark it as ON
Apply + Save + Test
Note: This may not work outside of the UKLoop Current - If step 1 didn't work for you, then you can try to change the Loop Current
In the WebGUI please navigate to: Expert Config > POTS > FXO Port Hardware Profiles > Loop Current Detect change from 0 to 300 or/and 500
Apply + Save + TestTone Detection - If the above two methods didn't work for you, then you can try to adjust the Tones.
In the WebGUI please navigate to: Expert Config > Tones > Tone Detection > Mark Enabled
Apply + Save + Test