First, there is not a released version of background tasks. The
services.appinventor.mit.edu is a test server for this capability, but it is not intended to be a final version. Apps may break using it. Second, because this is not a final version, not every component that could be used in the background can. There are a lot of changes that need to be made in order to accomplish this. My guess is that in your case, the PhoneCall component has not been updated to account for this change and the app crashes when you add it. I will forward your message to the person who has been working on services to see what can be done.
Evan