Complex sentence: Significant changes were planned to the way calls in and out of the London telephone area were to be handled that required exchange codes to be grouped together into sectors and it was not possible to implement these changes without a transition to all figure dialling.
Split into simpler sentences: Significant changes were planned to the way calls in and out of the London telephone area were to be handled that required exchange codes to be grouped together into sectors. It was not possible to implement these changes without a transition to all figure dialling.