GO Easy Buy does not require you to pay a fixed % of the retail price in cash in order to allow you to pay the rest monthly. In most cases, you will even end up paying €0 upfront.
You might in some cases have to pay an up-front difference (if there is one) between the total amount of your monthly payments and the retail price. Example: if your device costs €800, you will pay €750 through 30 monthly payments of €24.99 and the difference of €50 up front. If your device costs €750, you will pay €0 up front.