My account has been paid last month but Rain company says am owing them

Hello Pete kindly help me i have a problem with rain company i paid my Bill and they said it doesn't appear on their side i paid using their app i sent them my proof of payment but they keep on saying they don't see it and i personally went to their branch in *****o park and i show them my proof of payment they said they sent it to billing department and they will get back to me now am only getting the SMS and email that am overdue so kindly assist me thank you in advance