An apparatus comprising: a first transceiver arranged to communicate over a wireless network, the first transceiver comprising a first clock; and a second transceiver arranged to communicate other than by said wireless network, the second transceiver comprising a second clock. The second sends a request signal to the first transceiver. In response, the first transceiver transitions from a first mode to a second mode and provides to the second transceiver a response signal for calibrating the second clock relative to the first clock. In the first mode the first transceiver performs zero or more calibrations of the first clock relative to the wireless network, and in the second mode the first transceiver performs at least one additional calibration of the first clock relative to the wireless network, the response signal being based on the at least one additional calibration.