Billing and Airtime disappearance

Since I get exorbitant bill monthly, I decided that I will not pay my account for the month of April so that my line can be suspended then I will be able to see what exactly I've been billed for. In that month of April I was charged R1920 for PSD only. For that month only my bill was R2205. Last when I complained about my airtime no one has ever called me and call centre agents always say there are background apps running in the backround. My contract is on anytime 350, which means I should be getting airtime for R350. If you can check history you will see how many times my line was suspended. Account for March, June and July Amount is R570.87 since most of the time my account was suspended. Acoount for April I paid it in May. My question is where did my airtime go for the month since April to date (July). Currently account is up-to-date. I need to convert that airtime to data not even sure if it will help not get a suprise exorbitant bill end of the month. Cause last I was billed that PSD while I had 1G data bundle. The R570 amount I use to pay it was all inclusive when I was with Nashua and my line will be autolock when limit reach. MTN Billing mthod MMMHHHH