Modify VOIP program

I need an Voip application on Android platform. It should be an open source version or it can be something which you had done before. I will need one with as256 encryption algorithm, but similar methods would be acceptable. It can be either taken from elsewhere and put into the program itself or provided with an open source application. The task of this project is to help me to use it, to test it and to explain the code and document certain things as needed. Afterwards, I will need minor tweaking to be done to the program in regards to customization. Extension goals are possible. Preference is given for freelancers who can achieve this quite quickly and reliably (in a span of several days). Given estimated deadline will be taken into consideration and I will expect freelancer to complete project by that time.