MTN messages take airtime

To day my caretaker has put in R15 of airtime on her phone . Before she could use her airtime , MTN send her a lot off sms which toke all her airtime . <br> She does not want these sms which is send to her , and why does she has to pay with her airtime for these junk sms which is send to her . <br> The airtime is for her security to be reachable for her employer and her house .<br> I think this is outrageous that she has to pay for this junk sms 's , and this is not the first time and it is going on for months .