Telephone mute problem

[xanoreita]

My headunit is LG lac6710r (just some cheap HU I bought few years ago) and its been working fine untill now. Sometimes it just puts Telephone mute on and I can't turn it off unless I restart my cars engine, restarting HU won't help any and the phone mute wire isnt hooked anywhere and it never was. Is there anyway to get around this problem? Afterall its kinda annoying when it suddenly goes on. Can't find any button on the HU that would take it off either :s