This paper reports an experimental case study constructing an IP phone product line with the PLUS method. The IP phone product line equips a graphical user interface and interacts with the network. Its behavior is complicated, state dependent, and feature dependent. The original PLUS domain engineering process requires to perform dynamic interaction modeling among objects or finite state machine modeling for objects based on use case descriptions without any comprehensive view of the product line system behavior. However, use case descriptions are too abstract to capture the system behavior of the complicated system. To alleviate this abstraction level gap, we introduce system level finite state machine modeling to provide a comprehensive view describing the feature dependent behavior of the product line system as a finite state machine. That made use case descriptions sounder and dynamic interaction modeling and finite state machine modeling easier in our case study.