Have some pity on the customer service people. Give them some cues. If they call me, asking for my name, I say, "This is she." If no such cue has occurred naturally in the conversation and they "Sir" me, I say, "Pardon me, but it's ma'am, not sir." I knew in advance that I would not always have the patience for this, so I intentionally chose a name that is not particularly gendering (until I spell it): Renae. As a result of this choice, I can choose not to redirect the conversation or not, but being deep into transition and confident in my identity, I usually do. And, I have been working on my voice, which often makes the whole thing a non-issue. It's a great time to practice voice, incidentally. There is no pressure to speak above a crowd or project, and the listener's reactions give feedback on how you are doing.
Most customer service people are trained to try to make you feel at ease. Some are better at it than others, but they will at least try to gender you in the manner you indicated. You probably won't have to, but if they persist incorrectly, you do have the option of ending the call, with or without telling them why you are doing so. In the now very rare case I do, I tell them explicitly that I was quite clear about how I wanted to be addressed, and that if they cannot respect that, we have no further business to discuss.
None of what I said above is particularly helpful until/unless you are okay with being transgender. That is mission #1 in your transition, but so many folks here and elsewhere skip it, thinking that they can make it irrelevant or even "not true". Most of them drive themselves batty in the process, and push away those who really might want to help them.
