hopefully I can help you to some extent. I do understand what you're asking.
AND in terms of a quick fix unfortunately one doesn't exist. When I started I initially struggled to get numbers, then to get answers, then to get dates, then to have girls arrive at dates and so on. It is all a process imo.
When looking at flaking I think we should look at the several types separately. In your original message you talked about flaking in terms of a girl not finalising a date or time, this is the first type. Second type is making plans and then not showing or cancelling in the last minute.
I suppose for you the first step is to build attraction enough over the phone before you even suggest a catch up. Remembering that girls are emotion driven so a high interest girl when you meet her could start low interest when you have her on the phone..maybe she has had a bad day at work. You need to remind her of why she gave you her number in the first place and be the guy who can change her emotional state. Use references to when you met, tell her about how amazing your day was, tell her a story about how you took your mate with you to something and how they she had no idea (suggest competition).
Only once you have done this mention a catch up,, usually she will suggest it,. Ask what are you up to this week? She will usually ask back. Say something like, "work, dinners, coffees, lunches, gym. All the obligations, you know." Then perhaps say hey I've gotta go (insert activity here) but what are you doing thursday? Suggest a day you know she is free from when you asked her what she is doing. Then use either a direct approach or an inirect one like DonS suggested.
If she hesitates simply say, that's OK, nevermind. I'll talk to you next week or so. NEVER demand an answer but if she doesn't commit say never mind, i'll call you soon. Call a week later, say I have wednesday night free. If she doesn't do it the following week or skip a week.
There is a time to be assertive but not when making the initial date, maybe if she flakes on the date and cancels last minute..i've slept with several girls who i have taken this approach with in this situation but remember actions speak louder than words.