Increase in contract price

I upgraded my contract with Vodacom in May 2018 for a new phone, airtime and data. According to the written contract I would pay R327 per month for 24 months. I paid that amount the first month and thereafter they increased it with no explanation whatsoever first to R358 and later t R362. There is no Vodacom shop in our town and I have to travel 100km to a nearby town for Vodacom queries.