Abstract: Third party call control in a service management platform (SMP) of an enterprise system comprising a private branch exchange (PBX) in communication with the SMP over a trunk and line. Receiving a request over the line for communication involving a device. Invoking reliable transmission of provisional responses in the trunk. Receiving a receive only session description over the trunk. Invoking specific event notification between the SMP and PBX for calls answered between the device and PBX. Receiving notification of a call answered between the device and PBX. Responding to the request with the first session description as a send/receive offer. Receiving acknowledgement including a second send/receive answer session description over the line. Updating the trunk the second description as a send/receive update. Receiving a third session description with a successful response message from the PBX; the third session description being a send and receive copy of the first session description.