T-Mobile WiFi calling fails when using OpenDNS

[belawrence]

I found a previous thread regarding this issue, but didn't want to add to a thread with more than a year since the last post.

Anyway, after weeks of troubleshooting T-Mobile(USA) WiFi calling issues at my home, turns out using any other DNS servers besides OpenDNS solved the problem I was having.  My path to solution discovery was as follows:

- tried multiple config changes on my current router/firewall platform (pfSense)

- eventually ordered/installed a T-Mobile CellSpot - a T-Mobile branded Asus ac-1900 wireless router

- left it running with minimal config changes for one week - wifi radios disabled (I have Ubiquiti UniFi APs around the home),and port forwards to a security DVR system

- I then remembered that I use OpenDNS (no domains, services, or keywords blocked), so I set T-Mobile/Asus router to use 

- problems started again

- used alternate DNS server IP addresses - problem went away

- reinstalled pfSense w/alternate DNS server IP addresses - problem still yet to return

Is there anything specific to either T-Mobile or OpenDNS that anyone knows about that would be the cause of this?  I would hate to stop using OpenDNS, but understandably being able to make and receive phone calls is paramount.

[rotblitz]

I do not entirely understand what your exact problem is.

1. Can you post a link to this "previous thread regarding this issue"?
2. Can't you make or accept phone calls when using OpenDNS? 
3. If yes, what devices do you use for phone calls?  How are these connected to the router? 
4. Do you use exclusively VoIP or also a legacy phone line? 
5. Where do you have the OpenDNS resolver addresses configured?

[belawrence]

1: https://support.opendns.com/hc/en-us/community/posts/220010707-OpenDns-blocks-wifi-calling-on-t-mobile-cellspot-router

2: No - (android) phone gives one of following errors:  "REG90", "REG91", "REG99", "ER08", followed by either "DNS ERROR" or "CANNOT CONNECT"

3: see above (hint, it's called "wifi calling")

4: not sure the relevance, but home has no POTS service

5: a T-Mobile CellSpot (rebranded Asus RT-AC68U), then a Mini-ITX box running pfSense, and finally on the phone itself

[rotblitz]

Best is to follow the advises on the other thread to narrow this down, as follows:

As I understand, phone calls work with e.g. Google DNS or your ISP's DNS enabled, but not with OpenDNS enabled, right?  Now, OpenDNS does not do things different except that you can block DNS queries to certain domains.

Visit https://dashboard.opendns.com/stats/all/blockeddomains to possibly find related domains being blocked by your dashboard settings.  If you don't have a dashboard network at https://dashboard.opendns.com/settings/ configured, or do not have the correct IP address registered there, another OpenDNS user with the same ISP may have registered your IP address, so you'd use this other user's settings.

Also, what domains are being used by the T-Mobile phone system?  (They must use domains, i.e. DNS, else your phone apps on the Android devices would not report "DNS ERROR".)

You can copy & paste the complete plain text output of the following diagnostic commands to here, so that I get more ideas about what may be going on.

nslookup -type=txt debug.opendns.com.
nslookup DomainBeingUsedByT-Mobile.