i have been paying for 2 lines only using one

Today I learned that I have been paying for 2 lines for 17 months after removing my ex wife. Thats roughly $600. The person who removed my phone told me that my account would update and I got the notification that it did. But it wasnt the change of plan notice. It was just the phone removal and I didnt know the difference. Tmobile wants to pay me 3 of the 17 months which i find ridiculous. I have been a tmobile user since i returned from okinawa in 2014. 10 years. And now I find they they have robbed me of $600 and want me to be satisfied with 3 months credit. If this isnt resolved im obviously switching companies and taking whatever action I need to get my money back. Ridiculous.