Are you now living in the UK? Are you a Turkish national or a British national? If you are Turkish how long have you been living in the UK.
Getting credit is not just a matter of having a bank account. Vodafone have to be able to see that you have history of getting credit and being able to pay your bills when they come in.
If you have only recently arrived in this country Vodafone will not be able to see that and will not give you credit. There will be no way around it until you have been here longer.
What is the reason it is quite difficult i don't understand and won't understand because it is only Contract account and i have already given my debit card details. If i won't pay the bill then stop my calls. Is it so difficult ?
The risk to Vodafone is that you run up a massive bill, but have no money to pay for it — stopping you making future calls will not help with the costs already incurred. Because of this, Vodafone needs to be comfortable that you do not pose a credit risk before giving you a contract — as Jeff says, this generally means having some history, and, being in the country for only a month, that might be challenging 
There are some tricks, from what I've read online, to improve a credit rating, but I'm not sure whether these work or not.
