Messages doesn't work

I switched from Verizon to T-Mobile back in march and I noticed at the same time that I did that, Messages quit working on my Mac. It used to be linked so that I could communicate with iMessage users from my Mac if I wanted to, but that link between my phone and my computer was somehow severed. I've deleted my iCloud account and reattached it, I've done resets, but nothing has worked.

Has anyone else had this problem recently?

Works fine here and I'm on T-Mobile. When you send a message from your Mac to someone's iPhone, how are you sending it and what are you using as an address?