This case study examines the design of a new voice messaging user interface (UI C) to be compatible with both an existing UI (UI A) and standard guidelines. UI C was tested against UI A by having expert UI A users perform tasks to rate ease of use. UI C achieved ratings showing it was as easy to use as UI A for most tasks, including frequent tasks. While deviating from standards on some aspects, UI C aligned with them for 45% of aspects tested, and achieved the goal of being compatible with both the existing UI and standards. The study shows subjective user testing is important and that an overall positive assessment can result from mostly positive ratings of individual tasks.