It happens for instance if the phone is to my right and I listen with my left ear. So when I pick up the phone I use my right hand and transfer the phone to my left hand and left ear. During this journey the phone has to undergo a 90 degree anticlockwise rotation to change positions from the receiver facing downwards to facing towards the right where my left ear is (try this and it makes more sense). However, when Im done with the call I often pass the phone to my right hand with the receiver facing up (another 90 degree anticlockwise rotation). My right hand then continues the direction of this rotation to turn the phone back to a position where the receiver is once again facing downwards (another 180 degree clockwise rotation). If you follow that complete motion the chord has undergone one complete rotation of 360 degrees. The best way to understand this is to go through the motions slowly.
Obviously this situation works for either ear or in any direction, you just have to have the tendency to turn the phone in a consistent direction for the tangle to occur. Because the turn takes place over the course of a conversation it is very easy to convince yourself that it doesn’t occur, hence the confusion. If you rotate the phone on replacement in the opposite direction to which you pick the phone up, your phone cord will not tangle.
The argument that picking up a phone, stretching it without rotation, and then replacing it, will also result in tangling, is simply not true. Untangle a cord and stretch it ten times and you will see it does not happen without the accompanying rotation of the phone.