3CX recently rewrote the system prompt of its AI Receptionist. The old version was 450 lines — well structured, full of carefully considered policies — but it took up so much of the context window that the agent had less room to actually listen to the caller.

The core lesson: your AI agent does not need a manifesto. It needs a short, unambiguous spec.

Principles of prompt engineering for phone agents

  • Write instructions, not prose: clear, direct directives instead of explanatory paragraphs
  • Remove unnecessary definitions: don’t explain what a company or a customer is — the model knows
  • Avoid contradictions: two conflicting rules create unpredictable behavior; pick one rule per situation
  • Keep prompts concise: every line consumes context — less prompt means more memory for the live conversation

Telephony-specific watch-outs

  • Max AI Receptionist interaction: 3 minutes per call
  • Define fallback rules to a human agent for complex cases
  • Test with callers using varied accents
  • Set usage budgets via the OpenAI dashboard to control costs

The goal: an AI agent that handles standard cases efficiently and transfers complex ones intelligently — without ever leaving a caller stuck.

Source : https://www.3cx.com/blog/