Vodacom lock accounts that are paid up!

I am with Vodacom for basically 30 years. I always go to Vodaworld to upgrade my contract after it run out, but this last time Vodacom phone me and because I did not have the time to go to Vodaworld for the upgrade I decide to do my upgrade over the phone. Then when that contract started on the 1st of February all of a sudden my phone get locked because of credit limit bar that was never there before but my account is already paid on the 31st of January! The 31 of January they unlock it after I phone 082111. I went to Vodaworld the next day to fix the problem, and Vodaworld lift the credit limit to R3000. The 3rd of February they lock my account again! I phone Vodacom and ask them what the problem is and why do they lock my account but even the supervisor could not tell me why did they lock my account but she will get them to phone me the next day. I still wait for them to phone me? She unlock the phone. On the evening of the 4th of February they lock my phone again and again the supervisor on call could not told me why they did lock my phone?? He told me he will phone me on the 5th of February but do not unlock it??? What is the fault with Vodacom and why do they lock phones that are paid up????