PBX (Private Branch Exchange) systems evolve rapidly due to fast expanding services, ever changing communication technology, continuously evolving standards, and diverse communication laws and standards in different countries. As new features are added continuously, detection of feature interaction problems early in the development lifecycle and resolution of these problems through adaptable software are difficult challenges all PBX software engineers face. A feature-oriented reuse method FORM introduced in this paper concentrates on analyzing and modeling commonalities and differences in applications of a given domain in terms of features. The analysis results are then used for both the analysis of feature interactions and the engineering of domain-oriented software with a high level of adaptability (and reusability). We found that FORM facilitates analysis of feature interactions and variability (and commonality) of software before engineering and implementation start. With the understanding of the variability, adaptability and reusability could be built into software. The software architecture and the modules are implemented based the information hiding and encapsulation principles and could easily accommodate new features that have not been considered in the initial feature analysis.