My daughter had a similar problem. She has no data plan on her iPhone and she found that texts from some friends seemed to be delayed. Messages were not being s

My daughter had a similar problem. She has no data plan on her iPhone and she found that texts from some friends seemed to be delayed. Messages were not being sent/received until she connected to Wi-Fi. To fix, her friends who previously used iMessage to reach her had to start a new SMS conversation (or turn off iMessage entirely). If her friends just continued the conversations that started via iMessage, she could not get them via SMS.