well, BS yourself

You are basically correct in drawing an analogy there between an UDP datagram and a SMS message; both are just messages that are "sent there, godspeed", however, on top of both you could build reliable protocols, and the keyword here is the ACK.

When you send an SMS, and never receive any kind of confirmation, you are in the dark; it might be that the recipient got the message or not, you'll never know.

But if you get the confirmation of delivery message, you well can be 100% sure the message was received by the UE of the B-subscriber, and it signalled the RNC of its location area that the message has been received, loud and clear.

Now this problem here we are discussing breaks that paradigm; the message is received correctly by the UER-B signalling stack, and an acknowledge is sent to RNC counterpart. However, after that, the message is silently destroyed, never to be seen by the recipient.

This is a clear A-class pronto that should be corrected soonest. And if not, hurried by redhat-flag and 2nd-tier management escalation...