Enabling VoLTE & Wi-Fi calling on AT&T Lumia 640


    Hello Everyone,

    I thought I will share my recent success with everyone and provide some insight.


    First of all, especial thanks to folks from XDA Developers, Microsoft user community and helpful T-Mobile technical staff for helping me navigate through the issue.


    As I understood, successful IMS registration on a phone is what enables VoLTE and Wi-Fi calling. This requires three components:

    A.     Handset to support this feature

    B.      IMEI registered with carrier

    C.      SIM card to support this feature


    While VoLTE and Wi-Fi calling are 3GPP standards, most carriers do not enable by default for all handsets because (A) they want you to buy their handsets and (B) E-911 regulations require carriers to ensure that the service works reliably.


    This is primarily the reason as to why VoLTE does not work on many unlocked phones. This landscape is changing especially with CDMA carriers as they transition from CDMA to LTE (GSM) standard. At the end of their migration, carriers like Sprint (USA), Verizon (USA), Reliance Jio (India), China Telecom (China) will be LTE only, forcing them to provide better support for IMS registration process with unlocked phones.


    After buying phone from AT&T, I followed the instructions on AT&T site to unlock my Lumia 640. This followed by using Windows Device Recovery Tool to format the phone.


    I then followed the instructions posted on XDA site to de-brand Lumia 640. On step 11, I REPLACED THE COMMAND PROVIDED ON THE SITE, with the following:


    thor2 -mode uefiflash -ffufile "C:\Lumia\RM-1073\RM1073_02177.00000.15203.26115_RETAIL_prod_si gned_1001_023F70_TMO-US.ffu" -do_full_nvi_update -do_factory_reset


    This command allows the phone to use band 12, instead of just enabling it on user interface. I learnt this first hand when I travelled to an area where T-Mobile had 2G and LTE over band 12. I was on 2G the entire time.


    Once you have de-branded the phone, check if IMS registration is successful. The easiest way to know that is to navigate into settings>about and look for IMS registration field. If this says “unregistered”, then call T-Mobile and have your IMEI number registered.


    Companies like Apple and Google (Nexus), share IMEI numbers with carriers and hence the phone (auto) registration process is smoother. But Microsoft doesn’t care (or want) you to use their product and hence make the process more painful. This is primarily the reason as to why AT&T’s HD Voice works automatically on AT&T branded Lumia 950 but not on unlocked dual-sim Lumia 950.


    When you call T-Mobile do not tell them that it’s a de-branded Lumia 640 as they will simply hang up on you. Simply share IMEI number and the firmware number to make sure that you are on the latest firmware as supported by T-Mobile.


    Disable Wifi and restart the phone, this will force the phone to connect to T-Mobile network for all connections.


    If registering IMEI number doesn’t resolve the issue, then request for a new SIM card. T-Mobile will be happy to send you one for free.


    Once you get the new SIM (as in my case), disable Wifi restart the phone and it should work.


    Once that is complete, feel free to use “Windows Insider” app to upgrade to Windows Mobile 10.










    Lumia 640.jpg

      All replies

      • leicam

        Thanks for the post.


        You are pretty much spot on with the info. However, you left a key component (relating to WFC): OS.


        You can take a native TMo 640 running on 8.1, or a de-branded AT&T 640 running on TMo firmware and VoLTE activated, also running on 8.1 - WFC works without any issues on TMo SIM.


        Update the OS to W10M, and you will lose WFC - even with successful IMS registration.


        We have a lot of speculations here on why WFC doesn't work on 640 running W10M. Nothing concrete. Nothing conclusively points to Microsoft or Tmobile alone.


        Some of the FACTS we know are:

        1. You can run W10M release build on a TMo 521, which originally came with 8.0, later upgraded to 8.1, and WFC works just fine, but still via WFC app

        2. TMo branded Fierce One Touch XL running W10M has out-of-box OS native support for WFC


        So, no common denominator when it comes to why WFC doesn't work on lumia 640 on W10M.




        Now, saying what I said - I have a 640 on TMo firmware, OS updated to 14393.103, which is two builds newer than the latest non-insider public release 14393.67. WFC never worked, even with proper IMS registration. However, just recently I noticed that my phone was showing that WFC is activated - but it didn't survive reboot. Take a look at my screenshots.wp_ss_20160913_0001.pngwp_ss_20160913_0004.pngwp_ss_20160913_0005.png


        I really hope TMo allows WFC on this phone, even if they don't release W10M firmware.