The present invention discloses a system for implementing a software application programming interface (API) within a session initiation protocol (SIP) servlet. Such a system can utilize a state machine, a SIP servlet configured to execute the state machine, and an enterprise service bus (ESB). The state machine can be configured to send and receive a series of communications and can be defined in a definition document. The series of communications can represent the execution of a software function defined within an API. The SIP servlet can select a state machine based upon an indicator contained within a received SIP message. The ESB can be configured to execute a system function in response to communications with the state machine.