We're attempting to install an Extron DSP along with Shure ceiling mics and a pair of powered wall speakers in a small training room. The customer wanted us to use their existing #CP-8861 phone as the interface/dialer for making or receiving calls in the room. After confirming with Extron Support that this set-up should work, we completed the install. However, we continue to have issues with the audio, even though the DSP has effectively addressed most of the typical issues (echo cancellation, feedback, IP Telephony, tunnel effect, etc.) with this type of set-up. They (Extron Support) feels that the issue that persists may be due to the microphone on the Cisco phone not being muted, even though we have a line going into the mic input on the phone from the Extron DSP, which we are under the impression should disable the internal microphone. Any thoughts or assistance would be greatly appreciated.