Why isn't my iPhone receiving my messages from my S3?

A

AC Question

Iphone isn't receiving my messages from s3

I have a galaxy s3. My friend has an iphone 4 and it works perfectly and so does my phone. My friend can call me and I can call him. He can text me but when I send a text message to him he says he doesn't receive it or never gets them. On my end when I send it to him, it sends and says sent so I know it's being sent but why isn't he receiving them? I can send texts to other iphone users without a problem, 4,5,5c doesn't matter. He was able to receive my messages before but now he doesn't get them at all.