While training for a previous J-O-B, my supervisor had suggested to use a mirror placed on your back wall behind your phone. While I wasn't sure yet "why" I did it anyway trying to impress.
About a week later, I noticed this supervisor asking a co-worker if she had her mirror in place and thought I'd eavesdrop to get my "why". I did, and will never forget the reason behind this.
While talking to customers, we often forget that they cannot see us and we must portray a smile in our voice. Smiles can be heard through the phone lines and this has stuck with me. It is very true and I still use it in my home office to this day.
My clients, customers, etc. have no idea what kind of day I'm having. Nor do they really care. Regardless, everytime I look up while talking on the phone, it's a constant reminder to smile and my how the voice changes.