Sorry if this is old news, but I found at least a temporary workaround - my phone has been stable and functioning with wifi calling overnight, as well as my wife's phone (both LG v30s). The following steps are an amalgam of previous posts to some degree. IMPORTANT: per a chat with T-Mobile DO NOT bother with a factory reset, it is simply a waste of time. Per T-Mobile they say LG says it will not fix this.
There is only one goal here - change from IPV6V4 to IPV4V6.
1. Turn on wifi calling (no idea if this is necessary, but it worked in our case - another poster said to set wifi calling setting to prioritize cellular over wifi - I found that unnecessary)
2. Turn off all App Notifications (you'll restore them at the end) - I got the impression the pop up is being triggered every time an app tries to reach out looking for a notification. The phone will still be checking for texts, etc. but this may (or may not, but it worked for us) limit some of the triggering criteria. Settings --> Apps --> Notifications --> Disable all
3. Clear the cache and data for com.lge.ims.rcsprovider AND the much further down LG IMS app - Settings --> Apps --> App info --> Three dots button in upper right --> Show system --> click respective item --> storage --> clear cache/data (back / back / clear the second item)
4. After clearing the second item, timing is crucial here. Just click the phone dialer (don't bother closing out of settings) --> Dial 277634#*#, scroll down a bit --> click IMS Setting (if you get the pop up before you get here, you won't be able to see or click on the next bit, and will have to complete step 3 again) --> click Slot - 0 --> click IP version --> click IPV4V6
5. Go back to notifications and either select all, or use this as an opportunity to clean things up a bit.
I believe that's it. So far everything seems pretty normal, but not 100% sure.
Additional considerations:
This has worked well for us, but I haven't reset my phone, nor do I intend to unless it becomes necessary ;-).
Your menu structure may vary.
Our phones are about two or three years old, as are the sims, both running Android 9.
Mine is unlocked, wife's is probably still locked to T-Mobile.
Interestingly, my phone began having problems about ten hours before hers did.
The cellular connection at out house is terrible (which is why we NEED the wifi calling feature) - my wife's phone at one end of the house tends to connect to one local tower / my phone at the other end of the house connects to another (in fact, this whole issue feels like some infrastructure software change that is making it's way through the cellular system like a DNS library update...)
Another interesting thing I noted - in Wifi Calling settings, I see 'Error: REG99 "Unable to Connect" ' - this has to do with the 911 system being able to identify your physical address when you call, and in the case of wifi calling this is a problem. I know my E911 address is up-to-date on my T-Mobile account, but at some level this seems like it might be problematic now or in the future.
Lastly, the IMS service is related to VoLTE, so I suspect that the cellular provider - in this case T-Mobile and affiliates - simply made some progressive move forward, and left LG with it's somewhat abandoned / low-priority / poorly funded and managed phone support team scrambling to keep things afloat, and they have clearly failed. In the long run will things be properly repaired? Google LG's refrigerator line with the Linear technology (might check also for the term 'class action lawsuit'). Might also start shopping for a new phone...
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