Mobicell and Pep Cell

I bought a Mobicel cellphone v4 at Pepcell Newtown Junction on the 03/09/2019, I have used it until the 20th of December 2019, within 3 months of using it, it was freezing, and I called Mobicel support, and talked to Natasha, and she took me through the menu to reset my cellphone, whereas she is online, and she sad after we done is going to take me to original screen which it didn't and she sad if it doesn't take you, kindly return it to the shop, which I did on the 23rd of December 2019, and I have waited for them, no one called, until I call, is when they tell me lot of stories that I must pay, how I don't know, am I responsible for software? or my phone to be frozen?