how can you lose my phone?

They're human they make mistakes and a lot of times they do find the phone but no one ever posts a follow up.

Have you tried contacting Tmobile support using messenger on one of tmobile's social media platforms like Facebook or Twitter they also are a higher tier of support.

I was told by the T-Mobile rep that this happens only a few times and they are sure their system works well so I must not have returned the phone because they don’t make mistakes
 

From reading posts on this forum it seems to happen quite often.