You have three months left on your current contract when the supplier offers you an 18 month extension. You had your eyes on a particular latest model smartphone so look around for a similar plan with that smartphone. Your current supplier is offering a refurbished smartphone which is the model prior to the one you fancied.
Plan 1: 18 month contract, 300 inclusive talk minutes, 300 texts, 1GB data, you have a handset, payment is 20 per month. Getting the refurbished phone is -2 per month to you over the 18 months (-36 in total).
Plan 2: 24 month contract, 300 inclusive talk minutes, unlimited texts, 1GB data, 149.99 for the handset, payment is 25.50 per month. Having the latest smartphone is valued at +120 (worth an extra 5 a month in your eyes). There is a 60 payment for the last three months of the current contract (or cancellation fee).
The App shows that the lifetime cost of the new contract is a lot bigger than the extension being offered. You take the current suppliers offer, knowing that in 18 months you can shop around again.