\SIM not provisioned"24 hrs after conversion"""

I have been an MTN subscriber for 16+ years. In March of this year, I renewed my contract & received a new cellphone. All good. MTN made me a free offer during March to convert my SIM to LTE standard. Since my phone is very important to me for after-hours work as a standby resource, & for various after-hours messages from the systems I support, I delayed conversion until last weekend. I then took one of the 2 SIM cards provided & went through step-by-step conversion exercise & by 14h30 on Saturday afternoon 16 May, I was up & running. On Sunday afternoon at about 16h30, the phone connection went down with a \SIM not provisioned"message. I went to MTN Sandton City on Monday 18/05 at 11h00. Thami assisted & a call was raised as priority"