No one can solve a technical problem with WIFI calling !!!!

I've been trying to get WIFI calling (NOT whatsapp calling) working on my account for weeks without any success. The customer care consultants and technicians are all freindly and try to help but this is a technical issue that lies outside their area of expertise and competence and it seems like it is impossible to escalate a technical query to a level where someone with the right knowledge can fix the problem. I have literally made 10-20 calls to Vodacom customer care in the last two week, I've been to two Vodacom shops more than once each, and tried everything else I can to get this resolved, but no luck yet. Posting here as I hope it will be seen by someone at Vodacom that can pas it on to someone that can help. The short summary: WIFI calling works on my phone if I put my husband's simcard in my phone. He is on the same contract as I am (Smart M+). But with my SIM it does not work. The only difference that I know of is that I have a twin call service on my contract. Now recently I have been trying to get the twin call deactivated as it is also routing related and to make sure that it does not affect the WIFI calling, but now I got stuck with the twin call deactivation. It shows on the system that it has been done, but it is not fully deactivated when I test it. For example, an SMS from my primary phone to the twin call number gets delivered to the primary phone. I have had a back office query with reference number escalated to the Solutions Department to get the twin call deactivated (as neither the Customer Care or the Vodacom shop consultants/technicians had the authority or knew how to deactivate the twin call). The query has been "resolved" and closed, but the twin call is still not fully deactivated. And once it has been deactivated, then it is not for sure that the actual problem (WIFI calling) will be resolved. The last call to Customer Care suggested that I change my contract from Open Line to Top-up to get the twin call deactivated, since Top-up does not support the twin call service. But posting here before I do that as I am not even sure the twin call deactivation is the problem preventing the WIFI calling from working.