Before anyone in support tries to say it's because the sender's phone is not compatible, or the sender is using emoticons or whatever - this is NOT the case. I can get three or four text messages from my neighbor on her iPhone and then suddenly, the next one is all @#$#@$@%.
Same thing with my sister-in-law - she and I will be texting back and for the just fine - then gobbledygook. My friend ALSO has a flip phone - and her texts will be fine for days - then again: unreadable. A salesperson in the Verizon store reset my phone - didn't help.