Not all phones are created equal.  There are limitations with technology and SMS Snoop will only work on certain phones.  Apple do not allow this kind of technology to work on their iPhone and some phones do not have the ability to be programmed.  That said, SMS Snoop will still work on many, many phones.

If you have a Blackberry that is less than 3 years old, SMS Snoop will work for you (and even some that are older)

If you have a Nokia or other phone that uses the Symbian operating system (like some LG etc) that is less less than 3 years old, SMS Snoop will work for you (and even some that are older)

If you have a Windows Mobile device, touchscreen or otherwise, then SMS Snoop will work for you.

If you have an Android device, then SMS Snoop will work for you!

Remember, SMS Snoop is supplied with the understanding that the phone owner knows that it is installed