Call completion

Call completion is a telephony feature allowing some form of alternative interaction between parties who cannot converse directly with each other.
There are several possible factors which can prevent a telephone call from connecting successfully:

There are various definitions as to what exactly constitutes call completion.
Generally speaking, call completion may encompass the following services:

The rationale for providing these features is that allowing some sort of communication between parties unable to talk directly serves both the interests of the callers as well as the telecommunication operators, as the service allows them to get some return for providing the resources for the call.
Call completion features can be implemented on a private branch exchange, on a dedicated server
or directly on the client device.
There are several commercial companies which provide call completion features, as well as IETF documents specifying call completion features for open standards, such as Session Initiation Protocol.