Still being charged handset fee at month 25

I upgraded to the samsung S6 on 27.08.2015 in which i have been charged a handset fee ever since. Upon my next upgrade on 06.06.2017 they continued to charge me the handset fee of R307 for my old S6 stating that I was still liable for such for 24 months. I received my statement today which again shows a handset charge for my old phone for R307. Upon query with #111 they advised that my contract expired 1 September 2017 and therefore i had to pay the extra month of R307. Why am I paying for 25 months for a handset installment on a 24 month contract which had to expire on 27.08.2015. They said they needed to clarify the upgrade date in 2015 and as usual no response as yet. Does Vodacom usually ****** it clients in such manner?