According to the science of neurolinguistic programming, as much as 83% of the communicated message on the phone is the use of tonality. Tonality has to do with rate of speech, volume, pacing, pitch, and rhythm. The goal of tonality is to mirror and match (not imitate) the person you are speaking with.
For example, if you are speaking with someone who is talking a bit slower than you adjust your pace to reflect his or her pace. The voice is a tool to a salesperson just as a wrench is to a mechanic.
Learn to use this valuable tool. I have used a voice coach to both video and audio tape my phone calls and my classroom teaching. It was amazing to both listen to and watch myself. I would urge you to get a tape recorder and begin to record and debrief your phone calls.