time, or if a fall is detected. If a base station shares a phone line, it is able to terminate an in-progress call so that a call for help can be initiated over the telephone. With some systems, an alert arrives in the offices of the alert system operator (which may be a public rescue service or a private security company) and the data of the affected person (address, medical condition, family contacts) are displayed. With others, there is no system operator, and the user simply programs the numbers of family members, neighbors, or local emergency responders. If present, a responder can speak with the user through the microphone/speaker in order to clarify the type and severity of the emergency and discuss further measures. Depending on the organization of the service and the type of emergency help required, relatives or neighbors can be informed. If necessary, health care services or personal physicians can be notified or emergency medical services can be alarmed. Some monitoring services also provide the client with a USB medical alert device so that arriving emergency personnel can have immediate access to vital medical information. Some units can call user selected numbers, so relatives or neighbors can be called directly, avoiding the expense of a monitoring service. With some systems, it is common practice for the user to leave a house key with a neighbor or at the system office so that emergency personnel can enter the house even if the resident cannot open the door. Keys are kept in a safe and marked only with numbers so that improper use is precluded. In addition to this "active alarm" there is also the option of a "passive alarm" (sometimes called a "safety clock"), on the principle of a so-called dead man's switch. On some device is a button that the user is to activate several times a day; this confirms that the user is well. If this confirmation goes lacking for a longer period (usually around 12 hours), a telephone call is placed or someone is sent to check whether everything is in order at the residence. Depending on the company offering it, the system may provide for more than emergency use. Some solutions will periodically call to converse with the user. Monitoring In the event of an alarm, some systems will place a phone call to a community emergency service such as 911. Others will place