not carrier specific. happens across all carriers, just do a random internet search with your search engine of choice and you will find this issue across many carriers.
i think the short answer here is there is no way to get them in order on the phone side as they are coming in. the phone gets the messages as the system gets them.
sometimes i will get a "1/4" for example to mean that this is message one of 4, etc. not sure which carrier i see that from though, and even then the messages will be our of order.
sms messages are sent on an unused data channel in between voice and data packets so if there are some issues on the last mile to the handset (inter-carrier traffic, etc), this could affect SMS messages...