Parlay X was a set of standard Web service APIs for the telephone network (fixed and mobile).
It is defunct and now replaced by OneAPI, which is the current valid standard from the GSM association for Telecom third party API.
The APIs are deliberately high level abstractions and designed to be simple to use.
An application developer can, for example, invoke a single Web Service request to get the location of a mobile device or initiate a telephone call.
The APIs are defined using Web Service technology: interfaces are defined using WSDL 1.1 and conform with Web Services Interoperability (WS-I Basic Profile).