The invention relates to method of executing a service in a Media Gateway Control Function for establishing a call between a calling party's terminal served by a Mobile Switching Centre, and an IP Multimedia Subsystem Service. The Media Gateway Control Function is arranged to query a Service Control Point on which an overlay process is loaded. The correct SCP is identified by the MGCF using a correlation identifier originally published by the SCP itself after having received an IN service trigger from an MSC or GMSC. The MGCF will receive call related data from the SCP, such as the original dialled number. This data is used by the MGCF to populate information elements in a SIP Invite message. The invention gives a solution for providing GSM specific information, e.g. dialled number, location information, to an IMS service. This information may not be available to the IMS service through present methods.