I am not receiving every SMS from one of my friends. We are able to call each other, she gets all my messages and I receive some of hers, but not all of them. We are with different carriers and mine advised me to get a new SIM and reset my handset, but it has not fixed the problem.

It was suggested to me that her message settings may be set to a short validity period. Is it possible this is causing the problem? If so, how does she change this setting? She has an iPhone but I don't so I'd like to be able to explain to her how to go about it.