8 yr history depression. is my monotonous flat voice answering phones making my family business lose business?

It certainly could : I hope you are receiving good treatment from a psychologist and psychiatrist and that your depression will remit. Your flat voice could well make customers not interested in your business, and they may mistake this with a lack of courtesy and certainly a lack of enthusiasm for the business. I wish you well. Maybe you can get someone else to be receptionist while you heal?

It could impact on: How people react to your phone greeting. In addition to medication, work with a psychotherapist to help you feel better about life. You can also practice answering the phone with a more positive upbeat voice. Pretend you are an actor (or take an acting class) and try answering with a more animated voice. It might even help you feel a bit better. Good luck and let us know how you do.