Speak Clearly and Enunciate
Understanding what someone is saying on the phone fosters a productive conversation and helps to prevent a conversation filled with tension.
Whenever you are on the phone, speak clearly, enunciate and use simple words and phrases. Don't employ overly complex vocabulary; the last thing you want to do is confuse the customer on the line or make them feel inferior.
Failure to speak clearly and enunciate can cause people to often ask you to repeat yourself which can be frustrating for callers.