fixed Line rental charge changes month to month. why?

[#1]

On my bill, "Talk Evenings & Weekends Line Rent charge "
changes randomly each month from £13.99 to £14.99. why is this? there seems to be no pattern. it's not related to calling charges or anything as that's another payment.
thanks

[#2]

Hi laburnum5,
The increase in price happened on your most recent bill and is due to our recent price increase which we sent you a notification out in August.