My Sony Xperia XZ1 Compact (lilac) running a custom /e/ build supports VoLTE and WiFi Calling. Pretty sure that the unofficial /e/ build (latest is v0.17, v0.18 coming soon) supports them too.
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I’m using @petefoth’s build on the XZ1 Compact in the U.S. on an AT&T MVNO, and it does not have VoLTE or VoWiFi, probably because AT&T hasn’t whitelisted many older, carrier-unlocked devices.
However, when I insert a T-mobile SIM (unfortunately expired), I see that the setting to enable VoLTE then appears in the network settings, so I’m pretty sure VoLTE will work with T-mobile and T-mobile MVNOs. I plan to port away from AT&T in the next few months, so I’ll find out for sure. (AT&T will be retiring its 3G network in 2022, according to reports, meaning the XZ1C won’t work on AT&T’s network after that happens.)
List of approved devices (a PDF from AT&T’s website): https://www.att.com/idpassets/images/support/wireless/Devices-Working-on-ATT-Network.pdf
Can confirm that when using a Mint Mobile SIM card, VoLTE and WiFi Calls/Texts are functional on both the OnePlus 6T and Moto G7 Plus (REVVLRY).
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Sure can.
Purchased the G7+ preloaded and did the 6T myself. With some help.
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I’ve put your devices with carrier here for others who are looking to follow what you’ve done. 
@HellsBells
I have a Moto G7 Plus (river) flashed with /e/ 0.22-20220225166118 and I cannot make calls now that AT&T has turned down 3G. I have a VoLTE option in the mobile carrier settings and the Phone information in Testing (##4636##) says VoLTE is provisioned but whenever I attempt to make a call I get an instant “Call Ended”, hangup.
I’m wondering if I need an OEM modem update to make it work.
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If you’re sure that your model number is on AT&T’s approved list of devices, you should probably just contact AT&T (or your MVNO, if applicable) customer support. Maybe they can do something on their end.
Also, be aware of this: AT&T’s 3G Shutdown Appears To Be Causing Unintended Outages - BestMVNO
I’ve been on the phone with Redpocket several times for hours at a time and they aren’t able to fix this apparently.
“Outdated phone firmware” is where I am focused now. Was there an OTA update to the modem firmware on this phone that was missed because I had /e/ installed? or is the latest /e/ have the latest modem firmware for VoLTE (HD Voice) and I just need to wait for AT&T to fix it.
I can’t make calls, receive calls, retrieve voicemail until then. GRRRRR
Given that many other Android users seem to be experiencing similar problems with Red Pocket and AT&T since February 22nd, I tend to think it’s not a problem with the phone (although I’m no expert).
Some suggestions that may or may not help:
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If you have a SIM (active or expired) from a different MVNO available, delete the Red Pocket APN, shut down, insert the different SIM, power up, let the phone find the relevant APN. Then shut down, replace your Red Pocket SIM, power up, let the phone find the Red Pocket APN and provision itself. Then see if everything’s working.
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Switch to a different MVNO on AT&T’s network. See if it works.
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Get Red Pocket to switch you to their GSMT (T-mobile) network. You’ll need to pick up a new GSMT SIM. This assumes you have good T-mobile coverage and that your phone model passes T-mobile’s IMEI checker, of course.
@Taurus Thanks for that advice.
The RedPocket APN is another weird thing. While on the phone with the RP CSR she did a reprovisioning twice. Both times, removed the SIM, powered the phone off, waited 3 minutes, inserted the SIM, powered the phone up, and both times the APN that was added was named “PRODATA” and the CSR had me change it back to ERESELLER per their standard. Related? Maybe/maybe not. They clearly aren’t sure what the issue is.
I went to WalMart yesterday to buy a prepaid AT&T SIM for exactly that test but they were out. I’ll try again.
I don’t believe using a T-Mobile SIM is a valid test. Their 3G isn’t being shut down until July.
Can anybody confirm they are able to make calls on AT&T (or an AT&T MVNO) with /e/ Android 10?
It would demonstrate that VoLTE does in fact work on your phone, assuming it passes T-mobile’s IMEI checker. But agreed, a different AT&T network would further narrow down the source of the problem: RP or AT&T itself.
Here are more AT&T MVNOs: https://bestmvno.com/compare/att-mvnos/
@Taurus If I were to put a T-Mobile SIM in the phone, how can I know that VoLTE is in use? Having it enabled doesn’t ensure it’s in use, and that’s no different than how my phone appears on RedPocket now.
P.S. I was on T-Mobile with this phone back in October so I know it worked then.
If your phone doesn’t drop to 3G (H, H+) during a call, but instead stays on 4G/LTE, then you know VoLTE is in use.
But if you’ve already confirmed that on T-mobile before, no need to do it again.
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Related?
I suspect it’s related to the outages people were experiencing as reported by BestMVNO (see the link I posted above.
AT&T seems to have flubbed up their transition, but I’m only guessing.
I opened a bug anyway Moto G7 Plus (lake) XT1965-T unable to make calls after AT&T 3G shutdown. (#5063) · Issues · e / Backlog · GitLab
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After lots of reading… AT&T uses a non-standard version of VoLTE they call “HD Voice”. To use this there is a custom CSC update that AT&T would normally push to their branded phones, but cannot obviously to unlocked phone, or non-OEM phones.
Switching to T-Mobile is my only recourse.
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Or a cheaper MVNO that uses T-mobile’s network. 
Right… I’m on RedPocket AT&T now and have ordered RedPocket T-Mobile SIM cards. The MVNO doesn’t really matter. That T-Mobile doesn’t (supposedly) use a custom VoLTE and my phones should work, is key. Of course the T-Mobile 3G Shutdown is coming in July and I could end up right back here in the same predicament. I hope the /e/ Devs figure out how to get the AT&T CSC installed/fixed. I suppose it would require a carrier specific ROM, if in fact it is the CSC that is the problem.
Moving to T-Mobile still sucks regardless, because their coverage is bad in most places I go and doesn’t work at all in the datacenters where I work.
AT&T has microcells/repeaters/extenders installed in all those datacenters and it was perfectly wonderful. I’m hoping I can go back to it someday. Like this…
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