Hi pharmgirl0901,
Thank you for posting your question. As I understand it, your messages sent to other iPhone devices show a green bubble indicating they're sent as SMS. Is that right?
Also, to clarify, are you saying that at this time you only have one active plan set up for your iPhone?
If that's the case, it sounds most likely that you just need to make sure you're signed in to iMessage and that iMessage is turned on. Tap Settings > Messages > Send & Receive. Are you signed in with your Apple ID here? Also, under Settings > Messages, make sure that iMessage is on. This has more information about what you're seeing for reference: About iMessage and SMS/MMS
Feel free to reply back with an update if needed. Take care.