Need help to move back to Google Voice

Hello to All:

Recently switched over to a iPhone 15 pro max and for some reason my voicemail that was being routed to Google Voice is not working anymore.

It's now routed to the regular carrier voicemail system.  How do I switch it back and ensure that I can have all the functionallity working where unanswered, busy, unavailable, etc.. goes to my Google Voice System.

Someone mentioned to me about T-Mobile has a Conditional call forwarding (ccf) and I would need help on this but the iPhone 15 also has a live voicemail build into the phone. My current iOS on my Phone is version 17.1 

Hoping for some guidance on this as the Google Voice was very convienient. TY