You just need to have Node.js/npm and git installed.
NOTE: npm comes with node now.
Clone a copy of the main JsSIP git repo by running:
git clone https://github.com/versatica/JsSIP.git
Enter the directory and install the Node dependencies:
cd JsSIP && npm install
Make sure you have grunt installed by testing:
grunt -version
Finally, to get a complete version of JsSIP, type the following:
grunt
The built version of JsSIP will be available in the dist/ subdirectory in both flavors: normal and minified. Both linted (with JSHint).