It may be the battery, if this is going out it is a non replaceable battery. If the Plantronics Savor M1100 is still under warranty you should be able to return it. Warranty from Plantronics is 1 year. I have talked to several techs and have the same phone and headset. Maybe you should try resetting, pairing, subscribing it again.
Try deleting the headset out of the phone and if you can remove the battery in your phone. Once the phone reboots, try pairing the headset back up to the phone. See if this resolves your issue. If you go back to your original phone, does the headset sound fine?