Worth noting that SMS does not guarantee delivery in any way and can take unspecified amount of time to deliver. I've often seen messages delivered days or weeks after being sent.
SMS DRs are themselves SMSes, and subject to the same problems.
Remember how in the olden days, you sometimes got multiple copies of a single text? That was because the sender had marked "DR required", and some system along the way was dropping the receiver's DRs, so the SMSC kept retrying.
That's how all "reliable" protocols work when the message fits in a single transport unit. See the Two Generals Problem for why and how no protocol can tell you when to stop validating (though for practical purposes probability and the speed at which we can send messages allows us to proceed as if the problem were solvable)
