As easily as you can alter any applications that were written in Objective C. Which means that if you had to ask here, you can't. Even an experienced Objective C programmer would find it very difficult, if he could do it at all. (You have to know assembly.)
Hello, In case you are interested there is a sample program that descibes how to develop a VoIP client for iPhone (voip-sip-sdk.com/p_457-voip-iphone-integration-voip.html). I am trying to develop an own app according to it. Hope you will also find it useful.
First there’s a crash course on Objective-C in case you’re new to the language. Next there’s a tutorial series that will walk you through the process of creating an iPhone app from start to finish – using the most common APIs that almost every app uses.