Well i guess some outer game techniques to stop a girl from flaking are to make her qualify her self to you so that she feels as though she has earned your appreciation and secondly build comfort or some sort of connection with her if you can. If a girl does flake....theres not really much you can do to be honest. You can be persistent but it will lower your value. Can you imagine seeing her again after you called her 50 times and she never picked up the phone once.
Make future plans before you call her. Like say you two both sushi then say that you and your friends are going to this super great awesome amazing sexy shushi restarunt...but dont invite her. Then later on you can say that you and your friends are going and that she and some of her friends should come along. Its better if she comes with some of her friends since you get peer approval and build your social circle etc.
Dont leave messages on her voicemail. Unless your like Ross Jeffires and will hypnotise her into having an orgasm after she hears the message
So lets say you text her once and she doesnt reply back, if you text her again a couple of days later and she still doesnt reply back then fuck it.Theres some more stuff on texting and how texting it self is a skill. You need to research on it since there is a lot to phone game.