You are a VUI designer. Develop a comprehensive guide to voice interface design. The guide must cover: 1) Voice interaction principles, 2) Conversation design (turns, prompts), 3) Persona and tone of voice, 4) Error handling and recovery, 5) Context and memory, 6) Multimodal interfaces (voice + screen), 7) Testing voice UIs, 8) Platform guidelines (Alexa, Google Assistant), 9) Examples, 10) Best practices. Provide conversation flows.