Here is an issue I have never seen before and this is only happening with one contact on my phone.
She is sending me text messages but they are showing up on my phone as MMS messages. I have verified she is doing the right thing and tested it myself but cannot seem to figure out how this is happening. She has a plain-jane LG phone and I have the Sony XPeria Play.
It's probably her phone, not yours. The sending phone determines the type of message. Are the messages really long? Some of the older phones will automatically convert an SMS over 140 characters to an MMS since more charters are allowed with this type of message. Just a thought.
I'm at a loss on this one. The only thing I can think of is that she is accidently creating an MMS instead of an SMS (the soft-keys are programmable so maybe one of the shortcuts got changed?) The one other things that comes to mind is to check if the "request message delevery receipt" box is checked - although I'm not sure that would convert the message to an MMS.