It seems a bit trite to say “Smile on the phone, the customer can hear your smile”. Even though that is not literally the case, there is some truth to this.
Look back on any time you have talked to a person on the phone, whether it is just talking to a friend or family, or a colleague at work. Even though you were not there with them, if they were feeling upset or in a bad mood, you likely picked up on that a few words into the conversation. Now think on a time when you called a business that you call often, and the person on the other end did not answer in a cheerful tone. Now this in itself may not be because that a person is unhappy, but as the conversation goes on, you may pick up on small signals – quick short answers, a bothersome tone.
Think about your conversations on the phone or responses via email, and how your tone is coming across. The tone you present can often be misinterpreted as negative when you had not intended it to be that way at all. So can a customer really hear your smile? May be not, but they can pick up on and interpret the tone as less than friendly and may not likely to want to call again.