I wouldn't go in, I would call them. For the simple reason that they will have a dedicated customer retention team in their call centre, targetted on keeping customers with them. If you say you are considering leaving and do some homework first about the kind of deal you can get elsewhere and they are likely to do something to help in advance of the expiry of your contract. You might need to be quite direct and persuasive though ......... it's also helpful (I have found) to be able to quote your value to them as a customer ... how much do you spend with them a year etc...... good luck. Oh and also know what phone model you are wanting to upgrade to ...... I have a Samsung Galaxy S3 and can't speak highly enough of it.