Is MTN conning its "Pay as you Go" clients?

I am on "Pay as you Go" with my MTN phone. On more than one occasion whilst on a phone call the connection has been closed by MTN with a recorded message telling me my airtime is exhausted and to purchase more. On doing so and immediately checking my airtime balance thereafter I find that in fact at the time of the broken connection I had plenty of airtime left (on the last occasion this occurred over R50). On calling MTN I am told that the call will terminate when the airtime balance reaches R30 - not the R50 or sometimes more that I have had left in the past when this has happened. MTN did not provide me with an explanation but I requested they call me back ASAP and did so. In my view what appears to be happening is none other than commercial ***** on the part of MTN in making the user believe he or she needs to spend more money unnecessarily to buy unneeded airtime. In the absence of a full and proper explanation I shall continue to hold this view and consider making representations to the likes of the Consumer Ombudsman.