Senior Backend Developer – Vapi + Twilio + Python/PHP | Voice AI SaaS

We are looking for an experienced Senior Backend/Full-Stack Developer to join our team and take over the development and maintenance of an existing production Voice AI SaaS platform. Our platform, ReplyAI, provides AI-powered virtual receptionists for service-based businesses. The AI answers inbound calls, communicates with customers, captures information, handles enquiries, and can integrate with booking systems such as Google Calendar. The platform is already built and operational. Our current lead developer will be leaving the project shortly, so we need someone who can take ownership of the existing codebase and integrations quickly. This is not a greenfield project. We need someone comfortable entering an existing system, understanding how everything works, fixing issues, improving architecture, and continuing development. MUST-HAVE EXPERIENCE You should have strong hands-on experience with: * Vapi AI * Twilio Voice * Voice AI / conversational AI systems * Python * PHP * REST APIs * Webhooks * MySQL/PostgreSQL or similar databases * API authentication * OAuth 2.0 * Third-party API integrations * Server-side development * Production debugging * Git/GitHub * Linux/server environments Particularly important: You must have real Vapi experience. We are not looking for someone who has simply watched Vapi tutorials or experimented with a basic voice agent. You should understand things such as: * Vapi assistants * Vapi phone numbers * Server URLs / webhooks * Function/tool calling * Call events * Transcripts * Recordings * Call status events * Custom API integrations * Dynamic assistant configuration * Voice workflows * Debugging failed calls/webhooks * Connecting Vapi to external databases and APIs Vapi's server-side architecture allows voice agents to interact with external APIs, databases, and workflows, so understanding this type of integration is essential for this role. TWILIO EXPERIENCE Strong Twilio Voice experience is also required. You should be comfortable working with: * Twilio phone numbers * Incoming calls * Outbound calls where applicable * Voice webhooks * Call status callbacks * Call recordings * SMS where applicable * TwiML * Twilio REST API * Webhook authentication/security * Troubleshooting call routing and webhook failures Twilio's Voice infrastructure relies heavily on webhooks and status callbacks between Twilio and your application, so this experience is critical. WHAT YOU WILL BE DOING Initially, your main responsibility will be to take over the existing ReplyAI platform from our current developer. This will include: 1. Understand the existing system * Review the existing source code * Understand the backend architecture * Understand the database structure * Review API integrations * Review Vapi configuration * Review Twilio configuration * Review Google Calendar/OAuth integration * Review Stripe/billing integrations * Review webhooks * Review server infrastructure * Document the current architecture 2. Maintain the existing platform * Fix bugs * Investigate production issues * Monitor API integrations * Troubleshoot failed calls * Troubleshoot Vapi/Twilio webhooks * Improve reliability * Maintain existing functionality * Implement requested changes 3. Continue development We have several planned improvements, including: * Improved customer dashboard * Better onboarding * Improved AI agent configuration * Additional booking functionality * Improved call analytics * Better CRM functionality * Additional API integrations * Improved reporting * Automation improvements 4. Help us modernise the architecture We are also planning to redevelop the public-facing website using WordPress, while keeping the core ReplyAI backend/application infrastructure separate. The new WordPress website will communicate with our backend through secure APIs. OUR CURRENT / PLANNED STACK The exact architecture will be explained during the interview, but the environment includes technologies such as: * PHP * Python * MySQL/PostgreSQL * Vapi * Twilio * Google Calendar API * Google OAuth * Stripe * REST APIs * Webhooks * AI APIs * WordPress * Linux hosting/server infrastructure * Git/GitHub SECURITY IS IMPORTANT Because ReplyAI processes business and customer communication data, you must understand secure development practices. Experience with: * API authentication * OAuth * Secure API keys/secrets * Environment variables * HTTPS * Webhook validation * Database security * Access control * Multi-tenant applications * Logging and monitoring * Secure handling of customer data You will work with our existing developer to: 1. Review the existing codebase 2. Understand the architecture 3. Review the database 4. Review Vapi 5. Review Twilio 6. Review Google integrations 7. Review Stripe 8. Review webhooks 9. Review hosting/server configuration 10. Identify technical risks 11. Document the system 12. Identify immediate improvements The goal is for you to become comfortable enough with the platform to take over ongoing development. If you believe you are the right person, please tell us specifically what you have built with Vapi and Twilio. We are much more interested in real technical experience than a generic CV.