The paper introduces a new use for the B method as a means of specifying telecommunication services with the help of abstract machines and, consequently, it defines a formal framework for studying the feature interaction problem. An interaction is defined as a violation of an invariant and is detected when combining two or more abstract machines. The current B method is extended to allow the composition of abstract machines. The B method is supported by sofware that helps the specifier of services and features. We have not only modelled services within the B technology, but we have also extended the possibilities of B through the composition of abstract machines.