4 those who are having the issue is it all the time/everywhere. Or only in certain locations. If only in certain locations could be congested paging channels, that would also affect calls/missed calls etc. Or could be that the network thinks you are in a certain LAC/RAC and when being paged for the delivery of the SMS the serving cell is not set up for that LAC/RAC that the network thinks you are in.
Also anyone using iphones and if so if it is not already turn imessage off and see if that makes a diference