When a girl says "I'll let you know"

Hey peeps just trying to get an answer here, I texted a girl earlier on as the previous night we I had arranged to call her, basically I said "can I call you tomorrow night?" and she text back saying "yeah OK ;D"

So it comes to today like 11pm or so and I text her asking her if I can call her, and she replies basically "i will wake my little brother up if I pick up the phone sorry, he fell asleep in here"...err annoying when she said I could call tonight but I guess it couldn't be helped, anyway I text back saying "OK so when is good then?" and she texts back "I'll let you know buddy"

Now in past experience when girls have said they would let me know, they never, ever have..i hear nothing..so I really do not trust girls when they say this to me.

So peeps what do you think I should do? wait for her to 'let me know' which I'm 99.9% certain she won't...or what? anyone else been in this situation or similar?
When a girl says "I'll let you know"
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