A people monitoring method comprising: sending, from a portable device of a user, a request to a data facility via a network ; receiving, at the portable device and via the network, a ringtone audio signal in response to the request, the ringtone audio signal including a first audio watermark identifying at least one of the portable device or the user of the portable device; storing the ringtone audio signal in an audio data store of the portable device; determining , at the portable device , whether a first audio signal sensed by an acoustic sensor of the portable device satisfies a first trigger condition for emitting the first audio watermark identifying the at least one of the portable device or the user of the portable device, the first trigger condition being satisfied when a second watermark is detected in the first audio signal sensed by the acoustic sensor of the portable device ; and in response to detection of the second watermark in the first audio signal sensed by the acoustic sensor of the portable device and the portable device being in an operating state in which a ringtone is to be output: obtaining, from the audio data store, the ringtone audio signal including the first audio watermark identifying the at least one of the portable device or the user of the portable device; and applying the ringtone audio signal including the first audio watermark identifying the at least one of the portable device or the user of the portable device to an audio circuit that is to output an acoustic signal from the portable device. sending, from a portable device of a user, a request to a data facility via a network ; receiving, at the portable device and via the network, a ringtone audio signal in response to the request, the ringtone audio signal including a first audio watermark identifying at least one of the portable device or the user of the portable device; storing the ringtone audio signal in an audio data store of the portable device; determining , at the portable device , whether a first audio signal sensed by an acoustic sensor of the portable device satisfies a first trigger condition for emitting the first audio watermark identifying the at least one of the portable device or the user of the portable device, the first trigger condition being satisfied when a second watermark is detected in the first audio signal sensed by the acoustic sensor of the portable device ; and in response to detection of the second watermark in the first audio signal sensed by the acoustic sensor of the portable device and the portable device being in an operating state in which a ringtone is to be output: obtaining, from the audio data store, the ringtone audio signal including the first audio watermark identifying the at least one of the portable device or the user of the portable device; and applying the ringtone audio signal including the first audio watermark identifying the at least one of the portable device or the user of the portable device to an audio circuit that is to output an acoustic signal from the portable device. obtaining, from the audio data store, the ringtone audio signal including the first audio watermark identifying the at least one of the portable device or the user of the portable device; and applying the ringtone audio signal including the first audio watermark identifying the at least one of the portable device or the user of the portable device to an audio circuit that is to output an acoustic signal from the portable device. <EOS>