A device and a method for detecting presence of service on a telephone line prior to and during activation of alternative service on the line. The method comprises performing a first voltage detection on a subscriber line (S20), activating a subscriber line interface circuit (SLIC) if no pre-existing service is detected (S30), the SLIC providing an interface between the subscriber line and a service provider, and performing a second voltage detection on the subscriber line after a usage-dependent time has elapsed (S60), the time being determined by heuristics which consider several factors, including usage history of the line and time of day.