Do you know if the debt should be showing on my credit report if it is outstanding?

Hello. Iam new to this site and would please like some advice I went online the other night to obtain my credit score through Experian to see which outstanding debts i still have showing. There was two which i was aware off one which i did have red chasing me for regarding a HSBC debt which my sister settled with them but we have never recieved a comfrimation letter stating that this debt was settled? And the debt was paid over two months ago?? Apart from those two there was nothing else showing. I Have now recieved a letter this morning from red DCA regarding a T-mobile Debt i had around 2 and half years ago. This is the first letter ihave recieved regarding this amount and they stated that as i have not made any atemped to pay this they wanted me to contact them or they would arrange a home visit from balliffsto take goodsfrom my home or they would take me to court to obtain County Court Judgement. I was a bit puzzled as i thought that any outstanding amount that you had owing or that had not been settled would show on your credit history but this debt was not on mine should it show if it is outstanding??? . I am really worried and i am not sure what to do the letter was not very nice and did upset me. Any Advice please. Thankyou