Maybe this is an "Americanism" so to speak. It is a list of people in a group who call each other in an organized fashion.
A phone tree is a network of people organized in such a way that they can quickly and easily spread information amongst each other. Phone trees are especially common among parents, who often want to be able to communicate quickly about canceled events and other issues, and they are also used by activists and a variety of other groups as well. Setting up a solid phone tree takes time, but many people feel that it is well worth the effort, since the phone tree can be used again and again.
One easy to way to visualize a phone tree is as a phone pyramid. One person at the top of the pyramid calls two people, who each call two more people, and so on, until every person in the phone tree has been called. This allows people to distribute information quickly, without placing the burden of work on one person. As you might imagine, there are a number of advantages to setting up a phone tree, not least of which is the ability to distribute information quickly in a crisis.
Generally, a phone tree is organized by a single person, to eliminate confusion. The person organizing the phone tree collects information about all of the individuals who should be included in the phone tree. For example, a small rural school might have a phone tree which could be used to notify people about canceled school days and other events of relevance to the school.