Abstract: One-Number-Service (ONS) allows a subscriber to keep a single Directory Number when relocating to a different access point among one or more interconnected telecommunication systems. According to one aspect of the invention, a signaling packet for a call setup to a ported number is intercepted at an improved signal transfer point where a query to a database returns a new address of the exchange the number has ported to. The new address is used to update the signaling packet at the improved signal transfer point in order to set up the call to the ported exchange. According to another aspect of the invention, an improved exchange triggers to do a query to obtain the new address of the exchange the number has ported to. The new address is used to update the signaling packet at the exchange in order to set up the call to the ported exchange. Yet other aspects of the invention include combination of query and signaling packet processing at the signal transfer point and the exchange.