Mum's having a bit of a problem

[Hxc]

My mum is on O2 Contract, and has recently upgraded her phone from an oldish samsung to a new O2 Coccoon (I know, I know, she says it looks nice)

But now she isn't recieving texts from certain people, her sister (who is in the UK) and a friend in spain, texts are just not coming through. She's had the phone replaced and the same thing is happening, yet if she swaps her sim back to her old phone, there is absolutely no problem getting these texts.

Can anyone suggest what could be wrong?

Thanks