Billing after 24 months contract lapsed

I signed up for a 2 year contract with Nashua Mobile. I paid the contract for the duration of the 2 years. I lost my phone therefore I was no longer using the service as the agreed period had lapsed. No communication was received about my contract continuing or any outstanding fees. YEARS after my contract ended with Nashua I receive a call from Saya Recoveries that I owe them R2000 for a service that I didnt use. When I ask why I wasnt informed 3 months without paying, I 'm advised that there was some admin which was taking place, I am not prepared to pay for a service that I didnt use and after paying for the service I agreed on. Please assist as this might ruin my credit rating.