MTN Retention Department dishonesty

My MTN contract was due for an upgrade in October 2018, I started receiving calls every 2 weeks from an MTN consultant to encourage me to upgrade...every time they called I told them that I will not be upgrading until they give me the package and device that suits my usage and budget. These calls continued for months up until, after repeating myself to 4 different people about what I expect and want I finally got through to Portia Majola in the retention department. She was super helpful and apologized for all the inconvenience. We discussed my upgrade and I asked what we are going to do about all the unpleasantness - and she said that the Retention Dept can give me the first invoice off (after upgrading) through them. We agreed on this, discussed an appropriate package with the device that i wanted - Portia then gave me all the details for the upgrade and said that she would process it and let me know when it was done. I called back after 2 days, to check if she had processed the upgrade - but nothing was done. She said there was an error on the system, but she would sort it out and get the upgrade done, so again I waited. When I called Portia a day later, it was still not done - so we discussed the possibility of upgrading at an MTN store... Portia gave me the go-ahead, provided me with a Retention Reference Number to use in the store when upgrading and assured me that there wouldn't be any problems. I confirmed that by upgrading in-store, nothing would change and she confirmed. 2 weeks after upgrading, I checked my billing and saw that the agreed credit had NOT been passed as agreed. I contacted Portia in Retentions again to query, she firstly said that It will reflect for the end of May invoice, but later called me and said that because I upgraded in-store the credit (as we agreed upon in the first place) would not be passed as it was not done with her - Why then did she confirm that i can upgrade in-store? MTN says the right things to get you to upgrade but doesn't follow through once they have you bound to a new contract!