My airtime gone

I purchases 1 gig data and also purchases airtime I notice my airtime +- R300.00 gone The question is if i have no airtime i can't make phone calls and if i bought 1 gig data and i have to connect my data button so i can use data...but there is no warning or cut off if there is no data left just like airtime and there is no message to say your data is finished. ..i feel its ******* they they work...no data no problem lets deduct from airtime. ...but airtime its for me to nake calls and not for data thats why i bought 1 gig which cost me R149.00 Can you help me pleade Rsgard Hilda