Verify unlock request – AT&T will send a confirmation email. Click the confirmation link
Enter Unlock Code – Wait for ATT to email the unlock code then choose 1 of the following methods
With non AT&T SIM – if you have a SIM from any other carrier
Insert the NON AT&T SIM and turn the phone on
Wait for the unlock prompt and enter the unlock code
WithOUT NON AT&T SIM – if you DON'T have a SIM from any other carrier
Remove the AT&T SIM and turn the phone on
Navigate to phone dialer and enter ##7820#
Enter unlock code
SIM unlock is permanent, even after a hard reset
If AT&T emails you saying Your request for device unlock requires additional research to determine eligibility, just ignore it, you don't have to do anything. AT&T will email the code later
If ATT emails you saying your unlock request is denied. Just ignore the email and resubmit the request. You can resubmit with the exact same email. No one knows why this happens
How to de-brand ATT Lumia 640 LTE (Flash T-Mobile ROM) http://forum.xda-developers.com/w...e-t3294971
Credit: anubis23 (xda-developers)
Disclaimer: This method may brick your phone! Back up all data. I am not responsible if you break your phone. Use the steps below at your own discretion! Please be careful.
Supports all bands used by T-Mobile, including LTE band 12 (700bc).
For international use, 4 of 5 WCDMA bands are covered (missing 900MHz). LTE band 7 is supported, used in Europe and elsewhere, but with a lot of foreign LTE bands missing you should expect it to connect on WCDMA when overseas.
GSM network: 850MHz, 900MHz, 1800MHz, 1900MHz
WCDMA network: Band 1 (2100MHz), Band 2 (1900 MHz), Band 4 (1700/2100 MHz), Band 5 (850MHz)
LTE FDD network10: Band 17 (700MHz), Band 2 (1900MHz), Band 4 (1700/2100 MHz), Band 5 (850MHz), Band 7 (2600MHz), Band 28 (700MHz), Band 12 (700MHz)
Unlocking and flashing T-Mobile ROM will enable band 12 if you update the .nvi file. It is confirmed. (see post #337 on xda forum for commands to enable band 12)