The problem that you are having is definitely not a good sign. I would love to help you, but just a reminder, I am not a T-Mobile representative.
Since you made a rebate request for you iPhone, Im going to assume that you took advantage of the T-Mobile Carrier Freedom promotion. Were you able to talk to a T-Mobile representative, either on the phone, online messaging, at a store, or through social media, about your issues? If not, I suggest you start there first. This is a community forum, therefore help is always not available right away.
When you mentioned in bold:
How do I get T-Mobile to give me my rebate. I filled out the paperwork and sent in the required iPhone 6. I have tracking number for the iPhone I returned. T-Mobile lost the phone. Anybody else having these kinds of problems.
Does that mean the phone got lost IN TRANSIT to the warehouse, or did it get lost internally in T-Mobile's custody of the device? If it was lost during transit, the package carrier (UPS, FedEx, or USPS) would be responsible for this, and I'd suggest you start there as well.
Its good to hear that you are keeping some evidence of this issue. Please continue to do that, as that is a way to backup your claim.
I hope your problems are solved as quickly possible. I am very sorry for your inconvenience.
Have a great day!
We definitely need to take a closer look at where your phone is. It's never good for a phone to go MIA especially when $$ is tied to it. You'll want to get in touch with our Customer Care team via 611 or T-Force via Facebook/Twitter to open a ticket for our teams to investigate where your phone is. Give that a shot and let me know how it goes.