I AM HAVING TROUBLE WITH MTN

Hi,On Saturday (27 June 2015) I received an sms from mtn stating the ffg:Yello, this is a reminder that you have exceeded your Credit Limit of R 778.00 Your services have now been suspended for outgoing calls. Kind Regards,MTNSA.I went to mtn at the midlands mall in pmburg to query this sms . I was told by the consultant that I had made calls to the value of R1500,so I asked for an itemised bill but I was told that I have to call the call centre and request the itemised bill. At the store I had no choice but to make a payment of R700 so my phone may be reactivated because by this time all the functions on my phone were shut down when the sms clearly stated OUTGOING CALLS be suspended.Today (30 June 2015) I received another sms stating:Dear Mr Mangerah,As per our records your unbilled invoice is showing high DATA usage to the value of R 4083.***,for more information please us by dialing 808 from your cell or *** from a non MTN number.Kind Regards.MTN SP Credit Managemencontact.I also want to query this amount because I'm at work everyday & at work we have free unlimited wifi .All my downloads & updates are done at work with wifi.(incomplete,insifficient space)