Theoretically you should not be charged any cellular data if you are on airplane mode and you have not enabled WiFi Calling.
If you have enabled WiFi Calling which connects you to the XM/Verizon servers via your WiFi/Router/Internet then you may have cellular data charges for any MMS sent or received, voicemail, and maybe some system services, but should not be charged for any SMS (simple texts) or any other data.
When you start a new month of service you should reset the cellular statistics on your phone, and then you would see exactly what data usage you are being charged.
Look in your mobile data settings and it'll show you what used it.
Xfinity Mobile now has a Data Widget for Apple and Android devices. See their support page below: