Hey Chandra! I apologize for the inconvenience, but I am a little confused as to what activity you're seeing on your carrier's phone bill.
As a service, we are still only a third-party app on your phone. The phone number within the app is in no way tied to your SIM card or to your carrier. Your carrier (to the best of my knowledge) only shows activity with its own number (the number that you are paying for with your carrier)
If you are seeing calls activity on your carrier's bill tied to the TextNow number, it could be that you have the "voice fallback" setting turned on in TextNow's app settings (under "Calling"-> "Elastic Calling") that would then fall back to your voice network to complete the call if the data/Wi-Fi network is not strong enough to uphold it. This setting can be disabled at any point in time.
Please confirm what you are seeing with regards to "texts" activity, however, as all texts can only be conducted over an internet connection ( we can't fall back on your own carrier's SMS network)