Call changes are a method of ringing different changes by giving a set of commands ("calls") to a pair of bells to tell them to swap over.
The bells start by ringing rounds (1 2 3 4 5 6) until a nominated "conductor" issues a call. Example, the call "2 to 3" tells the second bell to ring after the 3rd bell (and by implication, the 3rd bell to ring after the first bell, so the change goes from 1 2 3 4 5 6 to 1 3 2 4 5 6. A further call could then be made by the conductor "4 to 5" instructing the 4th bell to ring after the 5th bell (and again by implication, telling the 5th bell to ring after the 3rd bell) so the change would go from 1 3 2 4 5 6 to 1 3 2 5 4 6.
The conductor continues to make calls until the bells are back in rounds. This type of ringing is typically used to fill in time (e.g. when waiting for the bride to arrive for a wedding or filling in time before the Sunday morning church service starts.
Unhelpfully - and unusually for an organisation that prides itself on standardisation - there are several (at the last count SEVEN) different ways of calling call changes and there is no particular geographical basis for the style of calling used, so one tower may call changes one way and a neighboring tower call changes in a completely different ways, leading to mutual incomprehension when the two groups try to ring together.