Have changed billing cycle without intimation

This query is regarding to the monthly bill received from the private mobile network VODAFONE Rajasthan.
Earlier I use to get the billing duration as 01 to 30/31 of the month but in September 2014, I got the billing period as 01/08/2014 to 31/08/2014 and 01/09/2014 to 18/09/2014.(i.e. for 48 days bill in September month)
I wish to know on what basis Vodafone changed the billing cycle as I never gave the request to them regarding to changing the same.
I am raising this query because I have to claim the bill in my organization month-wise and with this practice I am getting two bills of 48 days in September2014 month and 12 days bill in October 2014 respectively.
With this practice I am not able to get my entitled amount from my company.