Weird problem, texting someone about 8pm last night, no problem.

About 10.30 I was getting their texts, my phone said my replies were sending (using iphone no red exclamation mark) but they aren't getting them, and I have been texting a couple of other people and had replies no problem. I am also getting all of their texts through.

It's o2 to an orange phone. Any ideas? (Tried swtiching phone on and off at both ends)