Operation of DTMF mic on FT-857

[VK7PD]

I have 2 FT-857s and an MH-59 DTMF mic. I have setup menu 059 for 'RMT' on both radios; it sends the DTMF codes on one of them and not the other rig. It used to on the 2nd one but suddenly doesn't any more. It seems fairly unlikely that the rig would develop a fault stopping this function. I'm inclined to think that I've accidentally mis-configured something.
On the defective radio, the background LED lights when the DTMF buttons are pressed but it sends no tones. It works OK as a microphone.
Has anyone else come across this?
Peter VK7PD