I think the issue in this case is the therapist's blasé attitude and lack of concern in this matter. She clearly was not aware that her voicemail was unprotected and, rather than either saying (1) I will fix that right away or (2) it's not possible to fix due to technology constraints, she simply ignored the matter and never ever bothered to call back the OP-- who, at the time of the first message, was a concerned, paying client. The situation would be different if the OP lived in a town like the one Amandalouise describes, but she does not. Clearly, lack of funds or lack of available technology is not the issue in this case, with this T.

I agree with FeelTheBurn. Tell the therapist that if she chooses to ignore the problem, then you will report her so that the appropriate governing body can decide whether or not it violates any ethics laws. It's not the OP's job to determine whether or not the T is violating those laws; she can simply make the report and let the governing body decide. Clearly, the T did not warn the OP up front: "My voicemail is not secure; be careful when leaving a message." I'm sure the T's other clients would be surprised to know that their voicemails can be heard by anyone with T's phone number. I highly doubt they are aware of this concern. For most of us in the US, it's a "given" that voicemail is digital and password protected.

I just back from my grandma's funeral in a very, very, small, very, very rural town, of mostly low-income families (population 80! nearest "big" town 4 hours away!) and even there, everyone has cell phones with private, password protected voicemail. In 2013, a basic, $50 cell phone is not a "luxury" for the well-to-do. It's a basic requirement for most jobs and, quite frankly, it's a safety device. What if your car breaks down in the winter? How do you get help? My 86-year-old grandma, living on social security in a town of 80 people, had a (basic, inexpensive) cell phone simply so she could call for help, should she need to. I guess I learned something today-- that not every town in the US has access to these basic things. However, most do, and I think the expectation for those of us who live in other places wouldn't even think twice about voicemail being unsecure. I know I wouldn't. If I left a voicemail for my T, I would be shocked to discover that it was not password protected! I feel that is something she would NEED to tell me, since it is so far out of the norm for Ts practicing in my area (which is also fairly rural!)