Well, first test it. What are they sending you, and what phones does it show on correctly.
You may simply need to add a new charachterset to the phone. Perhaps they are sending moticons.
It could also be acommunication problem between different cell carriers.
Ordinary text should send between any phone just fine - the Envy 3 (I had the 1 and 2) should work just fine.
I can find character sets? I will do some exploring.