It is fully compatible (i.e., it can use all of the T-Mobile bands), but T-Mobile will not support it. By this, I mean that they will not help you beyond providing the appropriate settings and making basic suggestions if you have problems using your phone on the network. You must be self-supporting.
I'm not sure if WiFi calling will work. I hear that it will be baked into Android 6.0. You also might not have a functioning tethering app. Perhaps someone else can chime in here (possibly tidbits) regarding these functionalities that you normally do not get on a non TMO-branded phone.
Beyond that, calls, text and data should work fine.
band 1 / 2100 MHz IMT (Europe, Asia, Africa, Oceania, Brazil, India, Israel)
band 2 / 1900 MHz PCS (Americas)
band 3 / 1800 MHz DCS (Europe, Asia, Africa)
band 4 / 1700 MHz AWS (Americas)
band 5 / 850 MHz Cellular (Americas, Oceania, Brazil, Israel)
band 7 / 2600 MHz IMT-E (Canada, South America, Europe, Asia)
band 12 / 700 MHz Lower 700 (USA (AT&T, T-Mobile, regional))
band 13 / 750 MHz Upper 700 (USA (Verizon))
band 17 / 700 MHz Lower 700 (USA (AT&T))
band 20 / 800 MHz EU Digital Dividend (Europe)
band 25 / 1900 MHz PCS+G (USA (Sprint))
band 26 / 850 MHz Extended Cellular (USA (Sprint))
band 29 / 700 MHz Lower 700 (USA (AT&T))
band 41 / 2500 MHz BRS / EBS (USA (Sprint))
WCDMA (3G / 4G)
band 1 (I) / 2100 MHz IMT (Europe, Asia, Africa, Oceania, Brazil, India, Israel)
band 2 (II) / 1900 MHz PCS (Americas)
band 4 (IV) / 1700 MHz AWS (Americas)
band 5 (V) / 850 MHz Cellular (Americas, Oceania, Brazil, Israel)
band 8 (VIII) / 900 MHz (Europe, Asia, Africa)
CDMA (2G / 3G)
800 MHz / band 27 / BC10 ESMR (Americas (Sprint))
850 MHz / band 5 / BC0 Cellular (Americas, Oceania, Brazil, Israel)
1900 MHz / band 2 / BC1 PCS (Americas)
850 MHz / band 5 Cellular (Americas, Oceania, Brazil, Israel)
900 MHz / band 8 (Europe, Asia, Africa)
1800 MHz / band 3 DCS (Europe, Asia, Africa)
1900 MHz / band 2 PCS (Americas)
Good to hear! Thank you for your response!
what support issues?
I have had the Nexus 5X from release and everything worked other than band 12 which was removed then put back my Google within a month of each other. You don't really need band 12 at all to have everything working.
You are fine.
So WiFi calling works?
Just wanting to confirm, as I'm considering buying a Nexus the next time I upgrade (perhaps in a year or so).
Yes it works out of the box.
Visual Voicemail (the one in dialer)
Things I can't verify and more than likely not work.
Go-Go in flight messaging (I travel, but I sleep on planes)
RCS(Advanced messaging) new thing T-Mobile is starting to offer.
Visual Voicemail (the one in dialer)
That actually surprised me.
I use Google Voice for my voicemail and last week I got one of those spam "no ring 'marketing' voicemails" that popped up in my Visual Voicemail. I thought something went wrong until I realized it was probably one of those "let's spam all the boxes on a voicemail server" notifications since I never officially turned off T-Mobile voicemail.
The question should be: Does the Nexus 5X work on the T-Mobile network. In this case the answer is mostly YES. I am using the Nexus 5X since 4 days on the T-Mobile network and have come across two issues at this time. First let me list the operating conditions:
Phone: Nexus 5X, 32GB, Android 6.0.1, Patch level January 1. 2016, WiFi calling on
Problem 1: "Can't send message with T-Mobile, error-1"
This error shows up when a SMS is being sent and the phone is at this time connected to the internet on a local WiFi network. The phone can receive messages but is unable to send SMS messages. One must turn "WiFi calling" off in order to send the SMS. (one exception: the first time after an Android restart! Why, no clue)
Problem 2: Echo and lost voice when connected to a local network and WiFi calling is on.
This problem occasionally shows up when a call is placed and "WiFi calling" is on and the phone is connected to the local internet. It appear to depend on the pear connection setup. I observed this when a called a Verizon call center number. I had to turn "WiFi calling" off in order to have a successful conversation.
A last comment: I think T-Mobile should at least list a statement to the fact that the Nexus 5X is verified to use VoIP and the LTE band 12. The Google web site asks any potential buyer to check with the network provider if the capability is supported by the service provider. Therefore T-Mobile gives no answer and Google points to T-Mobile and we have the typical finger pointing procedure!
I don't have those problems and yes it works. Everything works.
WiFi calling (I guess you mean VIOP?)
Band 12 works(verified by T-mobile, and various people across the Internet)
Even voicemail in the dialer works(turn it on through phone settings)
Just for your information "WiFi calling" is the setting name on your Android phone!
VOIP is also VoLTE, using hangouts, Oovoo, and Skype for example. You do have to be more specific as the 5X does support those and one is natively installed.
After lot of research I cam across the setting causing the above symptoms. When I migrated from Nexus 5 to the Nexus 5X the setting CALL/CALLING ACCOUNTS/USE SIP CALLING was set to "for all calls". As a result one could observe the above symptoms. Also a forced WiFi call (wifi connected in airplane mode) eventually times out without any other action. After changing this setting to "for SIP calls only" WiFi calling woks normally.