About call directory extension capacity

I have some question about call directory extension

1. How much phone data can be saved to call directory extension?

2. How to get free space of call directory extension programally?

(Background)
I'm developing iOS app using call directory extension,
and I want to save phone data as much as possible.
But I don't know how much phone data can be saved and how to get free space programally. so, I can't handle saving operation proprorly.

Thanks for your help.