A method for determining the existence of calls in a VoIP system is disclosed. The antenna main body comprises an antenna element, female connectors provided at a first connector, a female screw section, and an engagement claw inserted into an antenna mount hole to make engagement with a lower side face of a roof panel. The impurity concentration peaks of the first semiconductor region include a first impurity concentration peak and a second impurity concentration peak being lower than the first impurity concentration peak.