All of my friends have iPhones with iMessage. When I compose an SMS message while they are using iMessage, it opens a new conversation and leaves the old one, also opening iMessage for me.
For my friends' iPhones, it automatically notices when we are both using iMessage and keeps them in the same conversation, but for me, it opens a new conversation entirely.
What is the reason for this behavior and how do I match it with my friends' iPhones'?