I installed some APs at a facility. This facility is now complaining they are having issues with their 2.4 phone system.
The APs that I installed (different SSID) are running but no clients are associated or transmitting data.
Is it possible to cause co-channel interference without data being transmitted? Thanks
Yes, 802.11 access points are chatty, users or no. You can expect every access point to transmit beacon frames on the order of 5-15 times per second.
These frames are transmitted very quickly and 2.4 GHz is generally very noisy, so I have difficulty believing that a 2.4 GHz phone system would fail in this scenario -- at least, assuming you didn't install an AP right on top of the phone system. Any device transmitting +20 dBm a few inches away from a device listening for -90 dBm signals could easily cause problems. Similarly, 2.4 GHz devices don't actually operate on 2.4 GHz the entire signal path; it's generally shifted down towards baseband at something like 100 MHz, and sometimes (particularly with high-power APs) this section is poorly shielded, and this leakage can cause issues even outside the target frequency band.
That said, none of that really matters for troubleshooting. The line of questions I would pursue is: does the problem go away if you shut off all your devices? If so, does it go away if you shut off one in particular? If so, what makes that one special?