Harassed by Vodacom

I have been with Vodacom since 2004 and within the last 4-5 years, had contracts with them. My account has never ever been in arrears. I recently upgraded my phone in August and changed from Top up 400 to Flexi 350. Now i am getting daily automated calls saying my account is in arrears and i must pay R160. I upgraded online and understood there was no fees. Also why can this amount not be automatically debited from my account when my payment is due? Why am i being harassed with 10 phone calls a day asking me when am I paying?? This is not the service I'm used to...Vodacom is making life very difficult