Group message replies

When I respond to a group message, only the original sender sees my response. The rest of the group does not. I have iPhone 6plus that does not have the option to switch group message “on”. Everything I have read suggests that this is a likely not a feature supported by my carrier Vodafone. I have spoken to them and they have said that the problem is that my phone is too old. Before I consider replacing what is a perfectly good working phone, would appreciate the input of the group. Thank you.