I have had this problem for weeks now and no one from Comcast Support has been able to figure it , i have talked to level 2 and so claimed level 3 and no one has any idea what to do, opened several tickets as well only recommendation they have is to re-install the app and install the app and to make sure you have internet ?? If my other apps works doesnt that mean i have internet? The solution they give is not working and also give false information that there was codes that they needed to put etc. Which is totally false What is going over there is there someone there that has good technical background is able to duplicate the issue at the time? .
Here are the details:
T-Mobile Cellular Network
This seems to just be happening over T-Mobile cellular data, if anyone has them as their cellphone provider please login , go to the dialer and see if you can make a call., NOTE: please have wi-fi disabled let it use your cellular data. See image for yourselft no one seems to believe that this is happening
Even though my situation is a bit different from yours, I am making this post in an effort to help you out. I have a Pixel XL first generation running the latest version of Android, my cell carrier is Google Project Fi which uses a combination of T-Mobile, Sprint, and US Cellular towers. I have an app that tells me at any point in time which providers tower I am connected to and so I know when I use the connect app if I am on T-Mobile or not. Additionally there are times when I am not in reach of any wifi signal and am running only on cell data. I can and have used the connect app to make and receive phone calls from my home number.
Due to the differences in our operating systems between my phone and yours, I won't be able to explain exactly how to check for the settings, but in another thread in this forum I made some replies to help people out with this same type of situation you are having. In very broad terms what I am trying to describe in that thread is how on an Android based system to check at both the operating system level and app level to verify that the options to conserve or restrict cell data isn't turned on as when those options are then there can be issues with trying to use the connect app.
Hope the information is of some help to you even though I can't explain how to check it out on your operating system.
Thank you for your intention at least you had the courage to reply, i did check those settings on my iPhone 7 and none of those recommend settings are there cellular data is not being conserved or anything from the system point. I also have an old android phone ASUS Zenfone 2E and the connect app over cellular data works there . i took my exisisting T-Mobile SIM Card and put on that phone with my same number and credentials and works there see screenshot it even registers and turn green next to my number . Logic points Then that the issue would point back to the iPhone/Apple device so i had a friend gave me his iPhone 6s with iOS 10.3.3 to test it out and still cant get it to work. There is something between T-Mobile and Apple that it doesnt like , because i used on Sprint and Verizon Cellular Data on an iPhone and the connect app works fine. What is special about this one that it doesnt work. I would like to see if anyone out there has any clues. Because comcast doesnt know and im beginning to think that they dont care because no one has gotten back to me.
Have same issue and opened trouble tickets. It got closed and asked to use wifi. My friends also have same issue, and don't see this is getting resolved. opened a FCC case as well and it is also getting very close, to getting closed :
All i also filed an FCC Complain because this all alot of users i did tons of research and did find a workaround by connecting to a free VPN "X-VPN" app connect your phone to it and the app will work via that connection, The issue is with T-Mobile serving IPV6 addresses and compatability problems (Android phones dont have this problem because they been under IPv6 for years ). Below is some part of the article. I Encourage everyone to share this and see if we can get some eyes noticed on Comcast to push on T-Mobile to fix this.
T-Mobile has continued to serve IPv4 addresses to iPhone users we may never know why Apple has not employed CLAT in the operating system (thereby forcing SIP application developers to embed this functionality in the app), or why T-Mobile decided to force IPv6 on iPhone owners despite this limitation. But the bottom line is that more than a few SIP applications have stopped working as a result