I could easily have changed my birth name to a well-accepted female variant simply by adding an A to the end of it, but I chose not to. I never liked the name, but even if I had liked it, I wouldn't have chosen to stick with it. I decided it would be best to have a complete change of name to something unmistakably feminine, something which could never be confused for a masculine name.
In my opinion, when trying to communicate with a person over the phone, that person may have already decided you might be a guy by the sound of your voice, and if you say your name is Lesley (Leslie) or Frances (Francis) for example, that person is forced to rely on their ears. In the early days of transition it is very important to be percieved as belonging to ones preferred correct gender.
My advice will always be to chose a feminine name that cannot be misheard as possibly being a masculine name.
I am fortunate, my voice passes very easily on the telephone as feminine. In fact, by sheer coincidence, a few moments ago our phone rang and I answered it. There was a woman on the other end who asked if I was Yvonne. Just in case I had heard it incorrectly I asked her to repeat the name but she hung up.