Support Center

Your texts include both the characters that make up what you’re saying and signaling information, like timestamps. However, when long texts are split up quickly (which they almost always are), the timestamps for each text may be the same. The receiving phone can’t differentiate between them. That’s why some broken-up messages arrive in a mixed-up order while others don't.

SMS is limited to 140 bytes (160 characters for 7-bit ASCII text). The GSM spec for SMS sets the message length at a maximum 140 bytes. So regardless of your carrier, if you try to send a text message longer than that, it has to be broken up into pieces. That in itself isn't a problem - the entire Internet works the same way The problem arises from two things:

SMS is typically sent over a packet network - that is, each piece is like a separate envelope in the mail with just a destination address. Each envelope is guaranteed to get to the destination, but it's not guaranteed to take the same path as all the other envelopes. So the order you drop the envelopes in the mailbox may not be the order they arrive in at the recipient's mailbox.

Each piece has its own timestamp, but the timestamp is just 7 bytes long (the standard is so old the term "byte" hadn't been invented yet, so they were called octets). Only one byte encodes the time in seconds, and it only does it in integer seconds (that is, all the extra values above 59 are never used).

This is a problem, as when a long text is broken up into 140 byte pieces, it's done almost instantly by computer and so each piece has the exact same timestamp. There is no way to put these pieces back in the correct order again.

When you send these pieces within a single carrier's network, there's a high probability each piece will take the exact same path, so they arrive in the correct order, and your phone reassembles them in the proper order. But when you're sending texts between carriers, there's a greater chance the pieces will arrive out of order, and you get a multi-part text whose parts are scrambled. Correcting this would require somehow encapsulating all the pieces of the text into a single packet, which means every carrier out there would have to agree on how to do this encapsulation. You might as well just create a new SMS standard at that point. (Which they kinda did with MMS - you can bypass this problem by setting the option for your phone to send long text messages as MMS. But unfortunately it doesn't work when people who don't have this option set send long text messages to you.)

Information found from (https://www.heymarket.com/blog/long-text-messages/)