If you used the return shipping label that should have been enclosed in the box your phone arrived in, then the return tracking number for that label is usually documented within your order. If you had to have a replacement label sent to you, it would have created a new return tracking number, which would have been placed in your order notes.
Sometimes UPS will create an 'alternate' tracking number to return a package to T-mobile and that alternate tracking number is usually documented, and linked on the UPS website along with information detailing the movement of the package with the original tracking number.
Have they done an investigation, called Handset Research, to find the phone yet? If not, you might want to try requesting that they do.
If you call in asking for a reprint of your device shipping label, what will happen is a new return tracking number would be generated, and Im having a hard time imagining how that would do anything but further complicate an already complicated situation.
If Handset Research has already been filed, and they could not locate the device I would ask for another investigation to be started.
All best - wtssup
Just wanted to check in and make sure everything is going alright!