I have a Partner ACS R8 system with 1 308EC module installed and 13 extensions on it; 12x 34D phones and 1x 16D.

Having issues with the microphones on the phones at certain extensions failing to work. It will be either really low volume, or not work at all regardless of the MIC mute button. Happens on both internal calls and external calls. Replacing the phone does not fix the problem, it stays with the extension. The problem comes and goes, but it is usually the same extensions affected. Strangely, in some of my testing/troubleshooting, I discovered that unplugging some phones appears to fix the issue. It doesn't seem to matter what phones are unplugged, just as long as some of them are. 3-4 unplugged usually takes care of it.

No other functionality of the phones are affected. Handsets work 100%.. It's just the speakerphone mic! The problem started a few months ago and keeps getting worse. There have been no wiring changes.

Anyone have any ideas?