Why isn't my phone number on tmobile updated to my other phone? How to fix message blocking error?

I got this iphone from my dad because he got a new phone. He let me use my grandmas old line and I got a new sim card. I tried texting and then getting this Message blocking error, i went on tmobile and the phone number is still on my grandmas old phone, why isn't it updated to my phone

Your my pal

Did you PORT the number to YOUR account? And are you leaving your GM's phone OFF, while trying to register your new phone on the network?

That's an issue to bring up with your phone company and/or clarify with your Dad.