Cisco Webex utilizes several tools to protect the cloud infrastructure and services from attacks and abuse, which could negatively impact customer meetings. As part of this effort, Cisco Webex makes use of third-party block-lists and compares the IP addresses from incoming calls (calls going to Webex from on-prem) against these block-lists.

If an incoming call matches a block-list, the call is rejected with a SIP response code of 599 with the reason text 'Incoming call denied'. Any subsequent retries of the same call, whether via the same IP addresses or not, will likewise be blocked.

To check if one of your IP addresses is hitting one of the block-lists, you can use the check utilities on the following page: 

• ScopServ International Inc.'s VoIP Block-list: https://voipbl.org/

Note: Only one IP address, or multiple IP addresses, may be blocked and result in calls failing even if they pass through different devices on different IP addresses.

Be sure to check all the Internet-routable IP addresses which your calls may pass through, and request unblocking via the checking tools on the above page. For example, if you are utilizing a Cisco Expressway-Edge (Expressway-E) cluster, check all the external IP addresses, typically referred to as the LAN2 IP address, or static NAT IP address. After the block-list provider has confirmed that your IP addresses have been unblocked, please allow up to one hour for any caching services to expire before attempting a new call.

If the issue persists, contact technical support. For help, see: WBX162 - How Do I Contact Webex Customer Services or Technical Support?