MTN Data Contracts

Have a data contract with MTN for 20Gb at R249 per month. MTN now have a special offer for 80Gb per month at R299 + SIM. When I tried to upgrade to this deal I was told I could not upgrade because my existing contract still has over 1 year to run and if I wanted 80Gb it would cost me over R1000 per month. I didn't think this was correct so I queried with MTN. I had a call back when I was with a client and was promised a call again 30 minutes later - still waiting. I then phoned the help line. Got to speak to a human after about 2 minutes of pre-recorded options and messages. The lady made note of my call reference number and asked me to "hang on while she checked". Ten minutes later listening to complete silence I hung up. I have not had a follow up call. The weirdest thing of all is that I then received an email from MTN asking me to rate their service. Don't know how to go forward from here but right now I am having to top up my data at R499 per 10Gb which is expensive and seems ridiculous when they offer 80Gb at under R500.