Why does iphone6 send SMS with traffic, and how to adjust it?

This is because the imessage function is used. When sending a short message, if one mobile phone has turned on the imessage function and the other mobile phone has also turned on imessage, it will send imessage by default. Turn off the imessage function as follows:

Open the settings selection in the mobile phone, and after entering the settings, select the information option;

After entering the information option, you can find the first imessage and click Close.