Billed for 2gb data and only 1gb was loaded

My 2GB recurring bundle was changed to a 1GB recurring bundle WITHOUT my authorization in March and I have been trying since then to get it sorted out and month in and month out it is not corrected even though i contact the call centre every time I receive my bill. And to top it off I get charged for 2GB this month and only 1GB was loaded. I have been a customer for nearly 15 years and this is the pathetic service I get. I will seriously think twice about renewing my contract with them when it comes up for renewal.