> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zeroruntime.ai/llms.txt
> Use this file to discover all available pages before exploring further.

# HD Voice

> Learn how to enable HD Voice in Zero Runtime SIP calls using the Opus wideband codec for crystal-clear audio.

HD Voice (High-Definition Voice) allows for crystal-clear audio quality in your SIP calls by utilizing wideband audio codecs. In the context of Zero Runtime, this is achieved through support for the **Opus** codec.

To experience HD Voice quality, the SIP trunk provider and the endpoints involved must support and negotiate high-quality codecs.

## Inbound Calls

For inbound calls (calls coming from your SIP provider to Zero Runtime), the audio quality depends on what your SIP trunk provider sends.

To enable HD Voice for inbound calls:

1. Ensure your SIP trunk provider supports HD Voice (e.g., Telnyx).
2. Configure your provider to use **Opus** as the preferred codec in their SIP invites.

If the incoming call offers Opus, Zero Runtime will negotiate it, resulting in high-definition audio.

## Outbound Calls

For outbound calls (calls initiated from Zero Runtime to a PSTN or SIP endpoint), Zero Runtime supports multiple codecs to ensure compatibility and quality.

By default, Zero Runtime sends the following codecs in the priority order:

1. **PCMA** (G.711 A-law)
2. **PCMU** (G.711 µ-law)
3. **Opus**

To enable HD Voice for outbound calls:

* You must ask your SIP provider to prioritize or support the **Opus** codec in their response.
* If the provider accepts Opus during the negotiation, the call will be established with HD Voice quality.