Customer has dozens of CME routers all running the same software. Just at only one parrticular (hence only on one particular CME router) site we have this problem.
When this 7921 wireless IP phone is communicating with any other phone within the site there is no problem at all. But when it is communicating to a PSTN phone there is a crackling sound on this 7921 phone. Customer says problem is reasonably easy to reprduce. The PSTN user hears everything clearly with no problem, but this 7921 phone hears crackling noise.
There are 8 x APs on this site and the problem happens on this one phone wherever it is in the building, ruling out any AP problem. Moreover there is no problem when this phone communicates to another phone on this CME router. So that eliminates the AP as well.. There are a total of 15 x 7921 phones and the problem happens on just this one physical handset.
So we changed the phone and that made no difference.
So we gave this phone another extension number and the problem continues to be with this one physical phone.
Other than change the phone again, I am not sure how to troubleshoot this problem.
Customer says calls where this phone to PSTN phone is when the crackling sound is heard by this 7921 phone.