The invention relates to method of executing a service in a Media Gateway Control Function (12) for establishing a call between a calling party's terminal (8) served by a Mobile Switching Center (2), and an IP Multimedia Subsystem Service (20). The Media Gateway Control Function (12) is arranged to query a Service Control Point (10) on which an overlay process is loaded. The correct SCP (10) is identified by the MGCF (12) 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.