MTN says we owe them R23 000

Hi, My wife just got an sms from Mrs DE KOCK. We have no record of your payment. Urgently fax proof thereof to ***. Norton Attorneys- MF074985 saying we owe R23 000, I phoned mtn and they say the account it u to date , now your lawyers are phoning no stop from *** saying we have to pay immediately, PLEASE HELP my wife's account number is A***, she pays her account on time every month