Data from Answering Services
Back in the 80's when I did some work supplying machines to answering services, I found that this doesn't always hold true. It seems that the "operators" had the ability to hide behind the telephone and project a different personality to the customers. I was able to observe the actual person behind the headset, and found that some (not all!) of the "nice" voices were (I need to be politically correct here) "weight challanged". It varied a bit but it seemed that physical attractiveness was not a job requirement, so these people were able to get the job on other abilities, a nice voice being one of them. As always YMMV.