CallNode + OpenAI GPT SMS/MMS Auto-Reply n8n Workflow

Instant AI SMS/MMS Auto-Replies from CallNode + OpenAI (n8n Workflow)

Turn incoming texts into fast, friendly AI responses. This CallNode + OpenAI GPT SMS/MMS Auto-Reply workflow for n8n automatically reads inbound messages (including optional image context), generates a short reply, and sends it back in the same SMS thread—optionally with an AI-generated image via MMS.

What this workflow does

• Receives inbound SMS/MMS from CallNode using a header-authenticated webhook.
• Understands attached media: if the message includes media URLs, OpenAI analyzes the image(s) and produces a brief description to add context for the reply.
• Generates a short AI response using a business-specific system prompt, keeping conversation context per sender/recipient with buffer memory.
• Sends the reply back to CallNode by returning the generated response as the webhook reply so it’s delivered into the same thread.
• Handles image requests: when the response includes an image request marker, OpenAI generates an image, converts it to base64, and sends it as MMS via the CallNode Messages API using HMAC-signed headers.
• Supports outbound texting via an n8n form: users can submit a phone number, message, and an optional image URL; the workflow signs and sends it via the CallNode Messages API.

Use cases

• Auto-replying to SMS inquiries for a SaaS, agency, or service desk.
• Handling MMS messages that include customer photos (e.g., “Here’s the issue—what now?”).
• Generating and sending visual content on-demand (e.g., “Send an image of …”) as part of the conversation.
• Operators sending quick outbound updates from an n8n form using CallNode.

Technical details

• Integrations: CallNode (inbound webhook + Messages API), OpenAI (chat, image analysis, image generation).
• n8n nodes shown in the workflow: webhook, form, crypto, if, set, sticky note.
• Security: webhook authentication via custom header name/value; outbound requests signed with HMAC.
• Deployment setup: create a CallNode number with messaging enabled and configure the inbound webhook URL + matching header authentication to the workflow’s production webhook URL.