In my case I work for FedEx and my plan is with Sprint. We are testing allowing customers to two way text with our virtual assistant and the responses are only coming out of order on Sprint user's phones. We have tested the top 3 carriers (still looking for a T-mobile user in my org) and the others all reassemble the messages in order and as one text. It has nothing to do with the SMS client on the phone (iOS or Android) or even the version of the OS.
It's my understanding our aggegator splits the messages when passing to the carriers and appends [1/3], [2/3], [3/3]. Seems like this would be the way to know what order is needed. Also, AT&T, Verizon, and T-Mobile all strip these numerations when displaying.