********* practice by cellphone service providers

I have a cellphone contract with Vodacom on the number shown below. Being a Transnet pensioner I need to manage my calls very carefully so that I don't exceed my monthly limit. I try very hard to carry time over from one month to the next. By the end of February 2018 I had managed to save 14 minutes which was carried over to March. During that month I keep within my allotted time thus retaining the 14 minutes I had accumulated. At the start of April I saw that I had only been credited with my contract allocation of 75 minutes. When I queried this I was told that the 14 minutes had expired. It was then explained to me that, only once ones monthly allocation expired, did the extra time come into use. I understand that this is usual practice for cellphone service providers. To me this is a subtle form of thievery as I have been ****** of time that I have paid for. To be fair and ethical I believe that the opposite should apply, in that extra time which legally belongs to the user should FIRST be used before going into ones contract time.