Unhappiness with Vodacom

I took out a cellphone contract on 30 July 2021 for 36 months the monthly instalment as stated on the contract to be debited from my account was set for R668.99. On 1st April the monthly instalment has increased to R685.00 I went to back to ask why the increase without notification only to be told that Vodacom has the authority to increase instalments without notice and gave me a number to call without and and email address. I fail to understand why staff in the branch are unable to assist and that the customer should go about there centres to try and get a reason for the increase . This has been a back and fourth without getting any resolution