Please resolve this issue

I signed up with vuma last year and I was paying everymonth until I lost my phone and I wasn't able to pay since the system they using sends an OTP number to my phone and I ddnt have access to my simcard.Tried calling them several time but they would refer me to different people until my airtime finishes without any assistance. Now their collections department is busy sending me messages threatening to blacklist me,still I try to call but no one is willing to assist.