ElevenLabs is renowned for its high-quality, realistic text-to-speech technology. They are increasingly offering agent-building capabilities that allow you to combine their superior voices with LLMs. To build AI voice agent with ElevenLabs agents step by step, you would typically:

Sign up for an ElevenLabs account and access their 'Agents' or 'Conversational AI' features.
Select or clone a pre-built agent template.
Integrate your chosen LLM (e.g., OpenAI's GPT models) by providing API keys.
Customize the agent's persona and knowledge base. You can even create AI voice agent with custom knowledge base PDF by uploading documents for the LLM to reference.
Choose a create AI voice agent with high quality ElevenLabs voice from their extensive library.
Test and deploy your agent.