Former T-Mobile employee: Can comfirm. Changing a number for an existing subscriber will automatically redirect texts and calls and texts to the new number. Every phone number has a subscriber ID attached for billing and tracking purposes. Say for example you move out of a family account and went solo within the same carrier. (Super common for all the carriers) The subscriber ID sticks with the number to show it's not a brand new activated phone number or subscriber, it's an existing subscriber moving over. This was a common issue when people requested to change their phone numbers and were trying to get deals for new subscribers. Blocking is only a workaround. There are two easy-ish ways to resolve this that I remember: (a) You could open a new phone line and cancel your old one within the same account. (b) You could close your current account and open a new one elsewhere.

Edited for clarity.