Searched the net, but nothing I found seemed to work. Turns out you apparently can't do it while it's plugged in. Bit weird.
Anyway press and hold the to little buttons maked with a dash - next to the make call / end call little phone icon buttons. Press and hold the reset button. Took a few attempts to get it to work though.
Press the Phone icon on the left (think they call this the send data button).