What makes a character strong? I don't think there's a single, straightforward answer to that. Are characters weak if they cry? Do they need to be physically strong? Do they need to be fearless?
I guess I feel there are many kinds of strength. There's physical strength, as in the power-lifting kind. Women are usually going to be at a slight disadvantage in this area because of simple size and muscle density reality. But there are many other factors to strength -- endurance, cleverness, determination, bravery. Bravery, in my mind, isn't being fearless. It's standing your ground or fighting in spite of fear, in a situation where you need to stand up for a good cause or a weaker person, or to fend off a bully. I found some cool surprises in this department in the book, when I examined it closely.
One thing I like about Red Phone Box is that all the female characters have their own brand of strength. They are in turns wily, agile, persistent, tough, and even physically strong within credible limits. Amber Goodman goes through a transition from being the relatively helpless girlfriend of a not so supportive guy to someone who confronts her own fears to save someone else. Cory is determined throughout the book, in spite of new challenges appearing at every turn. I also kind of love that she has... well, I don't want to spoil it, but by her nature, let's say, she's a fighter. (I am not a fighter, generally, but I admire those who can literally fight for a good cause.)
Gloria, in spite of being a spoiled rich girl, stands up when she's needed most.
Are there any weak female characters in the book? Well, there are some minor characters whose strength can't be ascertained. There's one evil female character, but she's no weaker than her male counterparts who take orders.
When we first started working on the book, I made a point of encouraging the creation of strong female characters. After a while I stopped thinking about whether it had happened and just went on with making the book what it is. I'm glad to see that on reflection, it's full of strong characters, both male and female. But it also has characters who are vulnerable, both men and women.