The implementation of Supplementary Services i.e. the way in which they should be invoked, is defined in Q932. However, Supplementary services are carried in layer 3 messages according to Q931 in the same way as all Layer three messages. These services augment or enhance Bearer services and Teleservices. They depend on the network for implementation, i.e. they may or may not be supported by the network.

Note on Supplementary Services:

A supplementary services may be included with a specific Teleservice. The same supplementary service may be included to enhance a bearer service which would not necessarily be associated with the same Teleservice. Thus, while supplementary services can only exist to augment Teleservices and Bearer services they are related to these services in an independent manner.

Supplementary services may be implemented in the following three ways:
(i) the keypad protocol.
(ii) the Feature key management protocol.
(iii) the functional protocol.
These are fully described in Recommendation Q932.