Why is my iPhone not notifying me as soon as I receive a message?

I'm not good at explaining but what I mean is like sometimes I get a text and it says it's been received like 35 minutes ago but it's only just come through on my phone… Anyone know why or what I can do to stop it? It doesn't happen all the time but it's been happening quite a bit the past few days and I'm not sure why but it's annoying because people think I'm ignoring them or they worry!

It's your service provider that causes that problem.