NO assistance with Wi-Fi calling problem

Utterly disgraceful service from MTN. I have been a business customer for around 20 years. I sent a lengthy email to Bus Customer Care describing the precise difficulty, and all the testing and attempts I have made to resolve it. All settings are correct on my phone, but Wi-Fi calling will not work on my sim card. I swop another MTN sim into my handset and it works perfectly. So the handset and settings are CLEARLY NOT THE PROBLEM. Absolutely critical to get this resolved, because the ONLY tower I can connect to is down during loadshedding, which means I can then get no OTPs, can't make any calls (have even had an emergency situation and been unable to call for help). Local MTN franchise couldn't help (phoned for tech support and waited half an hour in store for the return call that never came). Business Customer Care couldn't help by email. Simply told me to call 135, which is exactly the detestable course of action I had sought to avoid, knowing how ******* that service is. But I tried anyway. Hung on for ages with an endless message apologising for the delay before getting cut off. Absolute lowest of the low of customer service I have experienced anywhere, in any industry. Porting to Vodacom now seems my only option. Reference number: db7b7f05-b9a5-4a61-b58c-3d13c5fb52ea