Bad service

Good Day

I have a problem last month my remaining balance was 2400 so it happend that i had an emergency could pay but was adviced that if i skip one payment they add extra 150,was gonna pay dis month with that extra 150 now they have send people to my home with the letter saying the last tym i payed installement was on 2017 now they saying im owing them 5500 which is not fair cause i have all the sms thyv send with the remaining balance ,please help. im very upsent and dissapointed with their service