It should be Ms unless they've indicated a preference, which should then be respected. Hey, you never know, some women are Dr or Prof or Dame or Rev!
The historical meaning is simply that Ms was created to give women the same right to privacy that men complacently enjoy - is "Mr Smith" married? Who can tell? Not relevant. And so it was thought to be for women, when Ms was envisaged. This clearly hasn't worked as well as it should have if people still have misconceptions about it 40-50 years later.
But, honestly, it should be more relevant than ever - we're so far away from the nuclear family unit where Miss and Mrs were the only options (apart from the Widder Jones or Granny Weatherwax): single/never married, married, separated, divorced, co-habiting, same-sex civil partnership. Ms covers all of these possibilities with elegance.
And if it's not value-neutral it's really because a woman's identity is yet another thing that society reckons it has the right to control. Daily papers are forever refusing to call a woman by her stated title (and name, even) and call her "Mrs Husbandname". It may be just an image thing, but it's a very important image thing.
As you can probably guess I'm Ms, and I also prefer dealing with Mses in general business context. Besides handier, I feel it's more seriously minded. In the workplace, it's none of anybody's damned business if I'm "available" or not, and I feel Ms says this.
Miss or Mrs "based on age" is actually mirroring what the French and German speakers do. It's a widely-held principle so, even though I personally think it's crap, I reasonably sanguine if somebody from those ethnicities does it.
As to you, Mrs Fox, 
and anybody else who expects a big corporation to remember her title, I have expressed a preference, to a great many companies (including the Mail Preference Service and the Telephone Preference Service) not to get a lot of crapping sales calls and junk mail. Guess how successful that's been!... Grrr