Galaxy S4 - caller getting "modem" sound & I can't here them

[AC Question]

I have a Galaxy S4. When people call me they get a high pitched "modem/fax" type of sound and I cannot hear them. When I make a call (even my own v.m.) I hear the "connect" tone but no ringing or anyone answering. Again, all they hear is the high pitched sound. This was happening intermittently for a while and I was able to correct it with a restart. However, it appears to be on all incoming and outgoing calls. I have tried through a headset but it was the same results. All other sounds seem to be fine - music plays, notifications sound, etc.