We have had an influx of people reporting that their speakerphone has stoped working on 6408d+ set. The phone sets have been in place for about 5 years and I got about 10 reports last month of phones needing to be replaced because their speaker stopped working. Is it common for the speakers to go out after a certain period of time? or could we potential have some kind of power issue that is causing the speakers to go out? Thank you.