MY AIRTIME IS DISSAPPEARING

About three weeks ago, I started noticing that my airtime is disappearing. I would last know that I have airtime and find out that it has vanished sometimes without even making calls. When I press *141# to check my balance, I would notice that despite my airtime being gone, I still have internet bundles. When I try to surf through the internet and use social networks, the network would not allow me to. I will then load airtime again, then the social networks and internet would be operational, for a short while though. In other instances, I would load airtime lets say R15 and I would make one call and when I check my balance it would say I have R6 remaining, when I try to make another call with my remaining balance it says I have no funds available. This got worse to an extent that even if I have about R13 remaining on my balance Mtn would say that I have ran out of funds and therefore I cannot make calls. I have been trying and trying to call their call centre to no avail or they keep cutting my calls before I can even go through to the operator. This has been very frustrating as I have lost an estimate of R200 because of this and I cannot fully make use of my phone.