Thanks for using the forums to get an answer to your question, as the handset loan agreements are regulated by the FCA we do need to pass you on a credit check, no matter how long you've been with us or how squeaky clean your payment history is. Of course if you've had no missed payments its much more likely that you'll be accepted during this credit check! Which means you'll be on a higher tier credit class and will be eligible for the better, high ranking handsets.
Thanks for your update, I am sorry if that is the case ☹ we do also have lower tier contracts which are designed for customers with low credit, the phones may not be as flashy as the ones on the highest tiers, but they are really good value for money.
Plus, once you've completed the contract your credit class with us might even increase, so you can work your way up the credit ladder.