Snapchat doesn't allow you to save a voice call because it's contrary to its meaning. Snapchat's messages and photos destroy themselves after a user views them. This happens for voice call as well, meaning that you won't have the option ever implemented into the application.
Secondly, install a screen recorder and capture the screen and the audio to record the conversation, then you can save it as MP4.
Use the apps from the following list: http://android.informer.com/search/screen+recorder