I've bought this application. Well, it works as it says, but a few problems:
1. Difficult and confusing user interface. Need to press a lot of buttons before you can finish one operation.
2. The way of storing "previously" used prefix is dumb. And it just store 3 prefixes.
3. Slow in start up.
I found another similar application which is much easier to control. "Smart Dialler" under "Productivity". It provides an easy-to-use, one page interface for editing prefixes and selecting the phone number.