Calls not received

Over the last two months it has happened on numerous occasions that calls made to my number ( ********** ) is not received on my side - it will either divert to voicemail directly or in other cases nothing, at all, happens. In the latter case I may, from time-to-time, receive a SMS to inform me that I have missed a call (these calls does also not reflect in the "Call Log"). I have had system synchronization done on a number of occasions which seemingly solves the problem - I have, today, even been to a Vodashop where certain network settings were changed. These settings have never been changed and synchronization has never been necessary since I joined Vodacom some 20 years ago. What is happening here - this is extremely frustrating as I will never know when somebody is phoning and no business can be operated without being available to clients on demand.