ipad

You don't need an iPhone for iMessage and FaceTime (Video). The phone number on your SIM is linked to a data only service which is why the service cost a less than a phone line. Your iPad phone number is just an identifier for Tmobile network and not a "real" phone number which is why you don't have a phone app on an iPad. 

For voice calls, you can enable calls from your iPad if you link it to your iPhone and when you use FaceTime voice calls, it will show your iPhone number.

https://support.apple.com/guide/ipad/make-and-receive-phone-calls-ipadf97892b2/ipados

How do I delete my ipad from my account

call in and cancel that line?