The id that you should be asked for is the id that you were using in Settings > iCloud on it. For the id that you are trying to use does the phone show in the Find iPhone section of the account on http://www.icloud.com ?
If you were the original purchaser of the phone (i.e. it wasn’t bought used), you could try going to an Apple Store (make a Genius Bar appointment if you do) or try contacting iTunes Support online (https://www.apple.com/emea/support/itunes/contact.html) and see if they can remove the phone from the account - they are likely to want to see proof of ownership before they might help e.g. original receipt with the serial number on it