It looks as though the format for concatenated SMS’s are handset dependent - which may not be an issue for you if you are sending between wavecom devices.
There is a wikipedia article on Concatenated SMS - briefly it looks as though the messages are sent in PDU mode, rather than text mode; and a few bytes are used as a user data header (UDH) at the begining of each message. There’s a couple of links at the bottom of the page…
Here’s a link to a message on decoding the UDH - and this message implies that Concatenated SMS messages are documented in GSM 0340.
I don’t know what the SMS tarifs are like where you are - but for me it’s far cheaper to shift the data (even small amounts of data) via GPRS. Of course, if you need to initiate a connection to a mobile device if you can’t get a routable IP for your device there are a few issues (which aren’t insurmountable).
However, the big issue for SMS is that they are not ‘time delivery guaranteed’ - my best personal experience with an SMS is over 4 days between sending and receiving - and have a look at the year 2010/2016 bug that popped up in some Windows Mobile based smartphones over the new-year period.