T-mobile contract declined! Why?

About, two days ago I went to t-mobile retail shop to get an iPhone 5 Full Monty contract. Everything went normal. In that particular day, their system was down and the set up had to be done over the phone. As he was doing a credit check, he said it has been declined and I will only qualify for sim only deal. I asked the guy, he wasn't sure why and told me there could be various reasons.

I thought may be I had overdrawn my account few times that's why my card has been declined.

So, today I went again to get a contract on iPhone 5 but this time I went with my dad thinking to get it on his name. And surprise, It's declined again.

My dad never had any troubles with the payments and had enough money in his bank. We couldn't get any answers why it has been declined.

The address where I live is council flat and I have been living in the address for last 2 yrs. The letters for previous two tenants keeps coming to the address and we're still doing "Return to The Sender".
The letter looks like its from some sort of Credit collecting agency. I'm thinking is my address blacklisted because of the previous two tenants bad credit history. That's why our credit rating is bad because of the address. I discussed with my dad whether there was any missed payments or anything that could give us bad credit rating. There's nothing apart from the address.

I'm getting frustrated. Looks like we won't be getting any loans or finance in future.

Please suggest the right option or the actions I need to follow.

A Heartful Thank you in advance.

Added (1). BTW I have had an O2 contract for last 4 yrs now and it's gonna end this month. Never had any problems with O2.

Go to a different T mobile provider. It sounds like this particular store is the problem.

In the US, we have free access to our credit reports. If something like that exists in the UK, I would take advantage of it. It sounds like there may be incorrect information in your credit history.