When Edge Audio callback is enabled for one or more countries then Webex routes calls over the internet. If there are DNS, TCP, or TLS connectivity issues, or the call fails with a 380 or a 400-699 SIP error response, then Webex retries the call over a PSTN connection.

PSTN charges apply to failed Edge Audio calls that route over a PSTN network path. PSTN charges are billed based on your contract.

Retry call using PSTN Audio is enabled by default, but you can turn this off. Retry call using PSTN Audio triggers when the user selects the callback option and the call fails. Retry call using PSTN Audio doesn’t apply to dial-in scenarios.

SIP response to stop PSTN fallback

If you enable PSTN fallback on your site, any failed Edge Audio callback automatically attempts to fallback on Webex PSTN audio. If you detect a suspicious callback number and want to terminate the call, you can pass SIP response 608 to Webex to stop the PSTN fallback.

Disable Retry Call Using PSTN Audio from Site Administration

If you manage your Webex sites in Webex Site Administration, or have completed the Site Linking process, then you can disable retry call using PSTN Audio from Site Administration.

1

Sign in to Site Administration, go to Configuration > Common Site Settings > Audio Settings.

2

Under Callback settings uncheck the Retry Call using PSTN Audio check box.

3

Select Apply Settings.

Disable Retry Call Using PSTN Audio from Control Hub

If you set up and manage your Webex sites in Control Hub, then disable Retry call using PSTN Audio from in Control Hub.

1

From the customer view in https://admin.webex.com, go to Services and under Meeting select Sites.

2

Choose the site that you want to configure Edge Audio for, and choose Configure Site.

3

Choose Edge Audio.

4

Under Callback settings uncheck the Retry Call using PSTN Audio check box.

5

Select Apply Settings.

Number Blocking from Unified CM

From Unified CM Administration, you can specify which callback destinations use Edge Audio, and which callback destinations use a PSTN connection. If a call encounters a 380 or a 400-699 SIP error response then the call is retried using PSTN Audio.

If a call encounters a

  • 500-599 SIP error, then it tries all A records.

  • 380, 400-499, or 600-699 SIP error, then it won't retry all A records.

You can configure Edge Audio callback for only on network destinations. To do this, remove access to the PSTN route patterns on the trunk that you use for Edge Audio in Unified CM Administration. After this change, then Unified CM rejects all callback requests to off-network destinations, and retries the calls using PSTN Audio.

If you remove access to the PSTN route patterns from Unified CM, then you must update the blocking reason to something other than the default "No Error".

For example, if you want Edge Audio to route on network phone numbers to Unified CM and route off network phone numbers using a Webex PSTN, then make sure your Unified CM calling search space (CSS) doesn’t have access to the partitions with PSTN route patterns.