MTN demanding payment on prescribed account of deceased person.

Had a call from MTN or something demanding payment for an MTN account, when the relevant account belonged to my step dad that died over 6 years ago. And MTN is trying to demand payment from my mom, for an account that IS NOT her account, but my step dads. An account that by prescription act EXPIRED! They have no legal basis to claim that money, or put my mom through more trauma, demanding information and money).

Number that called is +27(0)10 ***.