Using formal specifications based on varieties of mathematical logic is becoming common in the process of designing and implementing software. Formal methods are usually intended to include all important details of the final system in the specification with the aim of proving that it possesses certain mathematical properties. In large, complex systems, this task requires sophisticated theorem proving, which can be difficult and complicated. Telecommunication systems are large and complex, making detailed formal specification impractical with current technology. However roughly formal "sketches" of the behaviours these services provide can be produced, and these can be very helpful in locating which service might be relevant to a given problem. Our case-based approach uses coarse-grained requirements specification sketches to outline the basic behaviour of the system's functional modules (called services), thereby allowing us to identify, reuse and adapt requirements (from cases stored in a library) to construct new cases. By using cases that have already been tested, integrated and implemented, less effort is needed to produce requirements specifications on a large scale. Using a hypothetical telecommunication system as our example, we shall show how comparatively simple logic can be used to capture coarse-grained behaviour and how a case-based approach benefits from this. The input from the examples is used both to identify the cases whose behaviour corresponds most closely to the designer's intentions and to adapt and finally verify the proposed solution against the examples.