T-Mobile only pulls your credit when you sign up as a postpaid customer (with no T-Mobile prepaid history*).
So, when you sign up with JUMP, your credit is pulled.
When you decide to upgrade through JUMP, your credit is not pulled again. They use your last known information and history with T-Mobile. Your down payments go down after a year of on time payments. So, on the front side, there's no benefit for low credit folks, but after a year, you're treated the same as high credit folks, provided you pay on time.
* If you have a year of on time payments with T-Mobile Prepaid, you're supposed to be able to get a postpaid account with the "zero down" down payments. I put "zero down" in quotes because the higher end devices still have down payments.
Just wanted to check in to to see if you needed any more clarification. Keep us posted if you need help.
Does this automatically happen or do I need to call in and set this up?
Are you wanting to set up JUMP on your account?
If so, you can do it by phone or when you're in a retail store when you upgrade.