Some calls not coming through

I have been using MTN for the past 15 years. I love the service and in general, the network. The only problem I have is for the past few months, some calls do not come through to me. For example, My wife (Vodacom), mother (MTN), Sister (MTN) and I are sitting together. When my wife or mother calls me, the phone rings. When my sister calls, her phone beeps and says "Number not valid". This happens with her, my brother, a VW service agent and Afrihost (as far as I know. I can't confirm who else has called and can't get through). This is a major issue and I just found out today that I missed a call for a job which I app**** for. I have called on many occasions and have done the following. 1. They reset the number. I removed the sim card and waited for the time they told me to and then reinserted it. 2. I tried this sim in another phone and I put another sim in this phone. The same issue continued on my sim in another phone. This obviously rules out the phone being faulty. Please assist