Cupon code detection, help with algorithm

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The telecommunication operator in my country has coupons that used to recharge the balance

I want to develop an Android application, that with the camera can take the scrached number and recognize and automatic recharge.

I think finding the number box, and latter preform an OCR or a pattern match in because are numbers only

I'm new in computer vision I set the environment and test some opencv ups, but I'm losing witch algorithms or sequence of algorithms may use to achieve that I want Exist segmentation, feature detection, blob detection, ... template match but the image maybe rotates scale because the use of the camera

And I'm lose I wonder if exist one algorithm to find the 4 extreme points of a coupon in one image from here I know how to proceed until the OCR part I don't want the code to do it of course What I want is a simple clue, answer like you may use X algorithmto have Y result

Thanks and sorry for my english