iOS Creating app based on open source code.

[eshroom]

Firstly, please excuse my ignorance as regards the use of open source code.
So, iSip (formerly SipPhone) is based on open source sip VoIP code. I want to make a similar app to iSip with some alterations in the way it works.
Am I able to use the code of iSip, is it publicly available?
If not, does anyone know where the open source code iSip's developers used is, so that I may build on that instead?

 

[robbieduncan]

Depending on the license you would have to make all of your code open source as well (for example if it's GPL code).