Keyless entry systems generate a signal to actuate a security system on a vehicle when keyless entry system is outside of a range of the vehicle. The circuit of the present invention includes a master clock input, a ratio control voltage input, a controlled duty cycle clock output, a high gain amplifier configured as an integrator having differential inputs, each equipped with a low pass filter, a controlled current source, a resettable timing capacitor, a threshold detector and a reference pulse generator. After receiving an INVITE message from a calling party, the SIP proxy negotiates with a transcoder for getting a list of codecs of the transcoder and their address information. The undercover functions to prevent an obstruction or a stone leaping during driving from directly hitting on a battery case.