An observation, not a disagreement:

While P.A.s are stereotypically female in culture (e.g. Most often in films, novels, TV shows). 'High-value' assistants in several cultural contexts are usually portrayed as male: maitre d's, butlers, concierges.

Unpack another layer or two - why do you interpret that to be more pleasant. It may be more socialized than you think at first glance. That's the problem with "for me"isms: you're just expressing climatized bias.

As a straight male, my answer is dead simple: female voices are attractive, pleasant, and comforting to me in large part because I experience romantic attraction exclusively towards women.

I can't speak for non-heterosexual men or for women on this question though.

How confident are you of this introspection? Might it not rather be (for example) that female voices are evocative of motherhood?

Speaking for myself, while I would prefer a high-pitched voice for a digital assistant (male or female - Data sounds friendlier than HAL, for instance), I would prefer it not to be sexualised. Can't say for sure until I've tried it, but I think it would be disturbing if a computer attempted to flirt with me.

I actually would agree with that, I'm just answering why it might be construed as a problem. Women's voices are also clearer in high noise environments.

Women's voices are often harder to understand for those who have lost sensitivity to high octaves. That is a typical pattern of hearing loss for older people, both women and men. Of course the ridiculous fashion in which some people choose to speak exacerbates this, but it would be a mistake to assume that such people aren't hired to answer phones.