Yikes, magenta2996096, that's an awful spot to be in. I hope your car's back to running smoothly! I know it's tough for our family to have cash on hand for those kind of unexpected expenses, so I can definitely identify with the situation you found yourself in.
To answer your direct question, the set of representatives you spoke with more recently are correct. We offer these extensions to give customers assistance when they need a little extra time to pay a balance, but once we've Set up a payment arrangement, it can't be changed or deleted. Though there might be an exception opportunity within 24 hours of the arrangement's initial creation if there's been a mistake (because our fingers all slip on the keyboard sometimes), if we advised otherwise during your calls in on the 22nd, we dropped the ball.
There should be account memos in our system that show a record of the conversations we had on the 22nd. A team with account access can review those notes, because we definitely want to try to make this right. Mixed messages and alternating replies are not the experience we want for our customers at any time, but especially not in a situation like this.
While we can't take a look at the account history here, if online communication is a good format for you, I would recommend reaching out to our T-Force team. They offer support via Twitter and Facebook, and in those channels we have a secure method to verify your account and can look at the notes and see what went wrong. You can copy and paste the link to this post an DM to @tmobilehelp or after clicking the "Message" button on the T-Mobile Facebook page (links to each of these are in my badge below if you're online), that way you won't have to repeat yourself and we can see how serious this is. I'm sorry to read that your previous experiences reaching out for help with this have left you feeling unassisted -- on top of everything else that's going on, that's awful. I hope you'll give us another chance to help!
So the General call center reps you spoke with are correct the terms of payment arraignments cannot changed or deleted once made, however, an I'll confirm with my wife; Financial care maybe able to help, I'll speak with her in the morning as she generally deal with them with regards to our account