IETF RFC 6910 PDF

The "completion of calls" feature defined in this specification allows the caller of a failed call to be notified when the callee becomes available to receive a call.

For the realization of a basic solution without queuing, this document references the usage of the dialog event package (RFC 4235) that is described as 'Automatic Redial' in "Session Initiation Protocol Service Examples" (RFC 5359).

For the realization of a more comprehensive solution with queuing, this document introduces an architecture for implementing these features in the Session Initiation Protocol where "completion of calls" implementations associated with the caller's and callee's endpoints cooperate to place the caller's request for completion of calls into a queue at the callee's endpoint; when a caller's request is ready to be serviced, re-attempt of the original, failed call is then made.

The architecture is designed to interoperate well with existing completion of calls solutions in other networks.