Doorbell 3 not working - mute!

Hi, I bought a doorbell 3 about 4 months ago. Around a month ago, the doorbell started to be mute when someone ring at the door i.e. they can hear me fine but I can't hear them. Yes I put the sound up and made sure nothing was muted on my side. The first time I solved it by taking the battery out and restarting the doorbell... 3-4 weeks later, the doorbell is doing it again. This is really painful and of course it does it when you are never home! any help someone could give or do I have a faulty device?? Thanks