request unlock code

I have been with tmobile for 4 or 5 yrs. I bought all my phones through tmobile site.

Now I want to request for sim card unlock codes for my phones, and tmobile is asking for proof of phone purchase. 

Unfortunately I was not able to find the receipt on my online account or the paper receipt. Tmobile declined to process my request.

Can someone from tmobile help me out?

Thanks,

Xin