N8N Cold SMS Engine: Slot-Based Sending with Quota Management

N8N Cold SMS Engine Architecture

Overview

Four interconnected N8N workflows that manage cold SMS outreach at scale through Signal House, backed by Supabase.

Flow

1. Fires at 9:00 AM — Daily scheduled trigger
2. Get leads — Pulls leads that need to be sent messages today
3. Determine slot — Checks if proper lead count is available for the sending slot
4. Check inventory — Routes leads by slot allocation
5. Pick leads — Select specific leads for this batch
6. Send message — Execute SMS send via Signal House
7. Log it — Record send status, timestamp, message content in Supabase
8. CTA failure handling — Handle failed sends with callback logic
9. Quota check — If more room in quota AND more leads available → loop back and send more
10. Complete — End when quota exhausted or all leads sent

Key Design Points

• All connected to Supabase backend for state management
• Supports pause/resume (can pause SMS sending at will without breaking the system)
• Handles ~1,000 new leads/day
• Uses Signal House for sending (cheaper for high volume outbound: ~7 cents/SMS)
• Reply classification handled by separate N8N workflow that routes to Mattermost (Slack alternative)

Pause Capability

• Need ability to pause SMS sending when sales team is unavailable
• Pausing should not break any workflows or lose leads in queue
• Leads should be held and sent in next available window

Why Not GHL Native for Agency Outbound

• GHL has messaging limits on native numbers
• Limits get hit daily for high-volume cold outreach
• Signal House bypasses these limits
• GHL native is fine for client sub-accounts (local service businesses with lower volume)