Arduino negative edge triggering for multiple inputs

I am using arduino mega with GSM sheild for triggering SMS. BUt have to code for Negative edge triggering to shoot single SMS. Can any one help me with that. Inttrupt option is out of scope cause I have to do it for 12 inputs and inttrupts are few.

Well it's a good idea to stay away from interrupts unless you need microsecond accuracy on the timing.

What have you done so far? Is there any code you can show us? Have you got it to send one SMS when one button is pushed?

Why do you think you need interrupts?
It makes code much easier to debug if you don't need to use interrupts.

What are the 12 inputs? Why do you need negative edge triggering?

Have you studied the pinChange interrupt capability?