Why is the volume dropping on calls?

When I am on the phone, the volume is dropping out on my calls so that the other person can only hear me very quietly. I can still hear them at full volume, and if I hang up and call back the problem fixes itself. Sometimes this happens after a long time, sometimes it will happen after only being on the phone for 40 seconds, equally it happens whether I am talking with the phone to my ear or on speaker. I cannot seem to establish any sort of pattern and do not know how to go about diagnosing/fixing the problem. Any ideas appreciated.