The Lookout Security will prevent anything from being installed via sideloading so that is the issue. And it can be frustrating since the Lookout Security is not descriptive about what it is doing.
So to install the special version of Boomerang version that has the Calls/Texts features, please disable Lookout security. Go to Android Settings, then search for Device Admin and uncheck for Lookout. Now try installing.
Then download the special version of Boomerang version that has the Calls/Texts features from: www.useboomerang.com/download and then tap open to sideload (agree to unknown sources when prompted.)