Just wanted to point out that it makes a difference whether the USB cable carries data or not. By default, the standards specifies that the USB port is supposed to deliver a maximum of 500mA over 5V. However, due to the popularity of USB ports, there were some deviation from the standard specification.
Some PC based USB port interfaces implement the "high power" type USB ports. These USB ports can deliver up to a maximum of 1.5A (or even more) and higher voltage and is dependent on the connected device to specify the power requirements over the data line. This is why devices like Blackberries and iPads will not charge over the standard USB port unless the device driver is installed.
In this instance, I am not sure which model/manufacturer of mobile phone the reader is using. However, I would think that during the phone call, some data has been transmitted to the audio device that happened to trigger some high power state and it caused over-voltage charging of the lithium ion battery in the mobile phone.
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