Here's what I can do for you, I am now submitting a request for a reset of your payment arrangement, once approved and processed you should be able to set up new one to be paid on your preferred date, sounds like a plan?

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Also, I see here that our request is now being process. while waiting I'd like you be aware that setting up a payment arrangement on my end would incur you $10. I would love for you to save $10 hence I am sending you a self- help option for free. Would this be okay for you?
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Perfect! To set up kindly follow this guide on your T-Mobile App.
Open the app and tap BILL.
Tap Set up a Payment Arrangement. 
Tap Payment Method
Tap Continue, read the Terms & Conditions and tap Agree & Submit.

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Also, I'd love to make sure I get to be of complete help to you, hence I would love to confirm if you have successfully set up a new one. Once done please do update me to check here from my end. Thank you as well!

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Our payment arrangement are strictly payable on the set date of installment, any early or late payment is credited on your bill and not on the payment arrangement hence the prompt to make a payment still today. But now worries, I am not leaving you hanging and we will correct it today for you.

TWO HOURS SYSTEM PROVISONAL TIME
This line update should take effect on your end within 2 hours system provisional and after a device restart, sounds good?