How to join received multipart SMS ( GSM Modem )( C# )

I'm programming an app that receives Short Messages (SMS) from a GSM Modem (ZTE Corporation) (+CMTI notifications).

I've already done my program but when there is a multi-part SMS I can not connect them to each other, and all of them come in separate messages.(e.g a 3 part message comes in 3 different messages). how can I connect them and understand which one is after which one? (Consider Someone Sends me two 3-part messages one after another, How can I distinguish these two messages?)

Programming language: C#
SMS in Text Mode (AT+CMGF=1)

Solution

I found it, by PDU Header I can find that if there is more smss and with user data header in that i can distinguish packets.