Do me a favor. Try answering the phone with a frown on your face, and then do it again with a smile on your face.
Your facial expressions can add a positive or negative effect on the way you sound over the phone. Your facial expressions transcend the phone line.
I'm sure you've heard something like that before; in fact, it's standard practice in many call center environments. There are several articles online which validate the importance of widening your smile.
There are even health benefits:
How to get into the habit of smiling on the phone
When I worked at the dealership, I had a mirror at my desk so that I could look at myself while on the phone with customers. This helped me form the habit of smiling and sounding positive. Give it a try!