Problem

Although Custom Conference Bridges (Organization Login > Settings > Organization > Notifications > Conference Bridges > Wizard +Add) are limited to 500 participants, sometimes when sending a Custom Conference Bridge Notification to less than 500 participants, participants may be unable to join the Conference Bridge via the Notification. 

The call result (Organization Login > Communications > Notifications > Active/History > {title of notification} > Delivery Details > Confirmed)  shows as:

Root Cause

When launching a Custom Conference Bridge Notification, the calculation of the number of participants (limited to 500) is done based on the number of voice call attempts. If participants have more than one phone path defined, the voice call attempts may be used up before the maximum number of 500 participants has been reached. Once 500 phone calls have been placed by the platform for the Notification, contacts from all subsequent phone calls (up to a maximum of 500 contacts) will need to dial in to join the Conference Bridge. For example, if a Notification is sent to 400 contacts via two phone paths with no delay between delivery methods, only the first 500 phone calls will allow the recipient to join from the Notification. This is because each recipient is receiving two phone calls for this notification which is 800 phone calls (300 over the 500 limit).

Workarounds

1. Include a delay between delivery methods so that not all phone calls are sent at once. This workaround will only work if most participants are joining from the first phone call.
2. A large Conference Bridge Notification can be split into multiple Notifications to allow for all participants to join the Conference Bridge from the Notification. In the example above, the notification could be split into two with one Notification going to 250 contacts and one Notification going to 150 contacts.
3. Dial in to join the Conference Bridge.