# Understanding timeouts and retries for the Amazon Chime SDK PTSN audio service
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The PSTN audio service interacts with AWS Lambda functions synchronously. Applications wait 5 seconds for AWS Lambda functions to respond before retrying an invocation. If no response is received after 20 second the PSTN Audio service will issue a [hang up](unexpected-hangups.md). When a function returns an error with one of the 4*XX* status codes, then by default the SIP media application only retries the invocation once. If you run out of retries, calls terminate with the `480 Unavailable` error code. For more information about AWS Lambda errors, see [Troubleshoot invocation issues in AWS Lambda](https://docs.aws.amazon.com/lambda/latest/dg/troubleshooting-invocation.html).