Arduino GSM2 click to android and iphone

I use Arduino Uno and a GSM2 click for sending and receiving SMS to an android phone and an iphone. To the android phone it works weel in both directions, but for the iphone it only works sending and I can not answere from the iphone.
So I just wonder what is different with the iphone vs android?
Have anyone some ideeas aboat that, please let me know.

PS: I have tested many programming examples with the same result.