Yes, this is definitely possible! You can create customized assistants for each realtor using the create assistant endpoint.
Vapi supports voicemail detection through both model-based and Twilio-based approaches which can you learn more about here
Realtors can use their own phone numbers using SIP or get Vapi-provided numbers.
Idea is to leverage Vapi's robust APIs while keeping the implementation manageable. Vapi's platform has the necessary components for building this type of service, including voicemail detection, customizable assistants, and phone number management.