Text messages frequently don't come through, or get delayed.
It's not a guaranteed service. When the sender's phone reports "message sent OK" all this means is that the message has got as far as the first SMSC which is promising to have a go at delivering. Some time, one day, if it gets round to it. It's no guarantee at all that it's going to reach the recipient.
There is no later feedback as to whether the delivery attempt succeeds or fails, so the sender has no way of knowing that it failed so maybe they should send it again.
There are other interesting features of the SMS protocol, most of which aren't implemented by either the phones or the networks. Just as an example, does your phone allow you to specify the expiry time on SMSs you send? If not, does it make up and send an expiry time of its own, and what is that I wonder? If a message is still in the network when its expiry time arrives it's just deleted.