the handset works because i can hear the caller and even on station to station calls you can hear yourself.
only on trunk calls is where you cant hear yourself but you can still have a conversation it is just kind of a nuisance.
Sidetone is sound picked up by your headset microphone that is then played back in real-time into the headset's speaker(s), acting as controlled feedback. To put it simply, it sounds like there is an echo of yourself in the headset. You don't always recognize when it's there, but you sure now when it NOT!
Back in the day before deregulation of the Telecom Industry the was something called "Systems and Practices" and the industry tried to maintain a 99.999% availability for their equipment.
That is less than 30 seconds of downtime per month! And now you have issues like you are having and much more!