Voice Fallback Plans

Overview

A voice fallback plan lets you configure backup voices that automatically activate if your primary voice provider fails mid-call. Instead of the call dropping or going silent, Sulus switches to the next voice in your fallback list, so the conversation continues without the caller ever noticing an outage.

You configure this via the fallbackPlan property on your assistant's voice object, listing fallback voices in priority order within the voices array.

When a voice failure occurs during a call, Sulus follows this sequence:

1. Detects the failure of the primary voice provider
2. Switches to the first fallback voice in your voices array
3. Continues sequentially through the list if subsequent failures occur
4. Terminates the call only if all voices in the plan have failed

Add the fallbackPlan property to your assistant's voice object and list fallback voices in the voices array:

{
  "voice": {
    "provider": "openai",
    "voiceId": "shimmer",
    "fallbackPlan": {
      "voices": [
        {
          "provider": "cartesia",
          "voiceId": "248be419-c632-4f23-adf1-5324ed7dbf1d"
        },
        {
          "provider": "11labs",
          "voiceId": "cgSgspJ2msm6clMCkdW9",
          "stability": 0.5,
          "similarityBoost": 0.75
        }
      ]
    }
  }
}

When using 11labs as a fallback voice, the following settings are available:

When using Cartesia as a fallback voice, the following settings are available:

• Use different providers across your fallback voices to protect against provider-wide outages
• Select voices with similar characteristics (tone, accent, gender) to maintain a consistent caller experience
• Sulus recommends configuring 2–3 fallbacks from different providers for optimal reliability

In summary, a fallback plan is a safety net for the voice layer of your assistant — it keeps calls running smoothly even if a primary voice provider has an outage, by automatically stepping through a prioritized list of backup voices.