GM300 & Ext DTMF Tone Pad Problem <HELP PLS>

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Hi all,

I am trying to get a GM300 to work with one of the cheap external DTMF pads that used to be used for intercepting your answer phone. The Pad works on various handhelds and Ham radio's OK, but for some reason when I try and send the DTMF audio into the Mic of the GM300 the Mic or radio appears to distort the DTMF tone, therefore the receiving station can’t decode the tones. On normal voice the radio sound fine with correct deviation etc.

Any help please....
Colin
G1IVG