I’m visiting other companies for work every now and then.
If they are in a fancy new steel-and-concrete office building with open space offices, chances are that cell reception is very bad. I once was in an office where I’m certain they had installed cell blockers on the toilets.
I read a couple of answers here, but I thought I’d give my 2 cents anyway. I mostly agree with the others that said it’s not a big deal and seems to be a principle of hers.
I saw that you’re not from the US. In Germany, there’s an unwritten rule that you never bring any kind of food or beverage to a restaurant. We have other fun stuff like most restaurants not giving out free water or charging you for using the toilet if you haven’t bought anything.
I could see her being upset with you if you drank your own water and ate your own food in the restaurant. But, again, that’s only from my German point of view.