Dial number with camera/picture
I don't think processing an image and extracting the number and calling it a better idea. the OCR processing will not be 100% all the time and it may call a different number at-times. 

A QR-code is more reliable than image processing and can be used for this if you dont want to type the number.

A QRcode Generator site to call to a specific number directly by scanning the code after generating it. Haven't tried it yet.