How can I know the protocol and develop a application for it?

In our country, there is a application called UUCall( an alternative of skype) , and I like it very much, but there is no android application for this. I intend to 开发者_运维百科develop by myself. My question is:

How can I know the communication protocol of it, and develop an application in android ?

Why can someone figure out and develop a application for skype/msn/icq in android ?

1. Put a packet sniffer in the way and look for common signatures.

2. Skype's protocol is closed, but as for the others...