### Answer The

agent_id

parameter in the List Calls endpoint is optional [(1) ]() . If you don't specify the

agent_id

in the

filter_criteria

, it will pull all call results across all agents in your account . You can use the

filter_criteria

object to optionally filter by

agent_id

like this :

json
{  
 "agent_id": ["agent_oBeDLoLOeuAbiuaMFXRtDOLriT12345"]
}

If you omit the

agent_id

from your filter criteria, the endpoint returns calls from all agents . Note: There was a recent bug where the

agent_id

filter was being ignored and returning calls from all agents regardless of the filter value [(2) ]() . This issue has been fixed .

​ To learn more, see the sources I considered: [[1] List Calls](https://docs.retellai.com/api-references/list-calls) [[2] Retell SDK /APInot filtering by agent id](https://discord.com/channels/1186424376300679209/1476277305352650752) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.

### Answer Based on the available sources, I don't have information about changing an agent's language for a specific outbound call via API before the call starts [(1) ]() [(2) ]() . The

language

parameter is configured at the agent level when creating or updating an agent . However, the sources don't show a way to override this setting for individual calls through the outbound call API. Alternative approach: You could create separate agents with different language configurations (e.g., one agent for English with

"language": "en-US"

and another for Spanish with

"language": "es-ES"

) [(1) ]() . Then, when making an outbound call, you would specify the appropriate

agent_id

for the desired language . If you need to dynamically switch languages during a call, one Discord discussion mentions using a transfer call tool to transfer to a different agent configured for another language [(3) ]() .

​ To learn more, see the sources I considered: [[1] Create Voice Agent](https://docs.retellai.com/api-references/create-agent) [[2] Get Voice Agent](https://docs.retellai.com/api-references/get-agent) [[3] hand off tasks or switch to another agent](https://discord.com/channels/1186424376300679209/1238523181824999445) ​

Feel free to tag @AI Evy with additional questions. I also appreciate feedback, it helps me improve.