Top up contract suddenly becomes open line

Back in September last year I took a tablet via Vodacom's online store. The contract I signed up for was a top up with 500MB free data. I do not feel comfortable with an open line as my 2 other contracts are also top up. Then problems all started when my account was unexpectedly debited by R2500 3 days before Christmas without even receiving an invoice. After enquiring, I was told that it was data charges on my open line contract. I know have an addition R5300 charge. As no documents were delivered with the device I have no proof. After my request for proof of contract choice the lines of the 2 devices I have paid every month while this query was on going hAve been suspended. I have not received any reply or proof them to date. (Emailing Sonny in the Data department). The call centre is of no help as I just transfered to the next agent and have to repeat my story over and over again. I feel conned, ****** and disgusted by the services of Vodacom.