Why does my phone not convert long sms messages to mms?

[roedel09]

Whenever I receive a long sms, over 160 characters, from an iphone user (and it only happens with this one person) it comes out as many small messages two lines or less. It's super frustrating because I've talked to the at&t people and they say it's an iphone problem. She's tried turning off imessage to see if it still does it and it did. Why doesn't my phone receive the long sms as one big mms message like it does with other long messages from people? Sometimes the messages are even out of order so it's impossible to text because some of our messages are really long. I have the galaxy s5 and she has the iphone 6.