No sound on my end of call.

When calling one contact in particular, they can hear me, but I can't hear them. Everything was fine and this started randomly. Sometimes the call goes through just fine, but often it doesn't. They have a 4s and I a 6, software is up to date on both. We're both on Wi-Fi networks that haven't changed. Have done everything shy of resetting network settings and restoring. I have seen alot of other people writing of similar problems, but can't figure out what this is since we can both call other people just fine as well as each other just fine sometimes.

Any help would be appreciated.