There's an obvious disadvantage / threat to privacy here, too, though. The logic is very simple.
Consider:
Why is the advert for women only operators/counsellors? The only possible reason is the recognition that female victims of abuse may be distressed by male operators/counsellors, right? And how could they distinguish a male operator? By his voice.
So, what makes any male applicant a woman? Solely that he perceives himself as one, and may or may not have gone through a predominantly administrative process, and, in the case of a small minority, superficial physical interventions, to formalise this feeling.
What difference does that male telephone operator/counsellor's feelings / paperwork / superficial physical changes make to the woman making the phone call? How does she even know of these feelings / paperwork / changes?
Unless you're assuming that only the tiny minority of trans women with voices indistinguishable from a woman's will be accepted (which I'd assume would be arbitrarily discriminatory by law, so surely can't be the case)...
...What is the difference, to the distressed female caller, between the male voice of a man, and the male voice of a trans woman? There is none. At all. How could they possibly distinguish? Both would be equally distressing. That this is the case is acknowledged in the advert not being open to men.
Except, perhaps there are a few differences...
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The distressed female caller may be more likely to feel under more pressure to suppress their trauma or discomfort to prioritise the self-perception of the male speaking, at the risk of being accused of bigotry and denied further help.
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The distressed female caller may feel uncomfortably conscious that the male speaking is prepared to transgress female boundaries, and thereby experience the call as a secondary violation, of sorts.
Now, I think if the company is open that it employs transwomen, or men, that mitigates this somewhat, in enabling callers to make an informed choice about whether to use a helpline that comes with the risk of encountering the shock of a male voice. But I still think 1) It's electing to exclude a potentially quite significant proportion of vulnerable female victims at the expense of employees' preferences, and 2) Frankly, I cant imagine they are open about this - what crisis phoneline includes #callsmaybeansweredbytranswomen on its toilet door stickers? This, too, I imagine, may leave them vulnerable to legal attack.
TLDR: This advert is saying, "We recognise many female victims of abuse would be distressed by speaking to a male, but if that male thinks about himself in a certain way, they should set aside this distress."