Would it be worth asking her if she has any other ideas that might work for both of you? (Validate her concern about her need for a phone. ..while trying to problem solve collaboratively). Or, offering her an alternative solution that might work for you as well? Maybe she Is panicked and can't see any other solutions? Can you show her that you can stay calm and offer a possible solution in a collaborative (not condescending) way?
And just a personal note, the way my H would phrase things was always a trigger for me...he would say, "why don't you..." in a (to me) demanding way, which for some reason drove me nuts. It's a small thing, but saying "what if you..." or "is it possible for you to..." would have sounded better to me.
