In the old times all sms were transport of the tcap layer in ss7 stack. Tcap was reliable non flow controlled protocol with no way to handle the extra traffic(carriers just used 64kb trunk line) by all the new kids sending sms. Then somebody in the vendor community had the idea let’s drop sms if the trunk are too busy. The problem the protocol had no way to notify the sender of the drop message. You can imagine the path of the following hacks to solve that new problem