Speakerphone issue

If I switch to speakerphone while already in a call I lose all sound, but the call continues and the person I am talking to can still hear me. If I try to switch speakerphone off I've still got no sound through the earpiece.
If I initiate the call in speakerphone it's no problem. I can switch speakerphone off if I want without a hiccup.
Any ideas what the cause could be? I've tried disabling Voice Enhancement and Voice Privacy but the problem persists.