[One (M9) Q&A, Help & Troubleshooting] jollywhitefoot:
I had this issue last week as well. It came on out of the blue and did all the usual AT&T troubleshooting and technical support. As of Monday, though, I think I have the issue fixed. I don't know if AT&T made a network modification that spurred the issue or if I changed something on my phone. Anyway, I'm not sure exactly what step along the way actually fixed my issue, but here's what I did.

1) Go into mobile data settings and see if turning off "Enhanced 4G LTE services" helps

2) I'm not sure if ARHD has this option, but if you can set your phone disable 4g in mobile data settings, turn 4g off and see if you can receive calls...I bet you can (so that's a temporary solution)

3) Alternatively, you can dial *#*#4636#*#* which will bring you into phone settings. Go into phone information, scroll down and on preferred network type, change that from GSM/WCDMA/LTE auto to GSM or GSM Auto (again this is just a temp fix because basically you're disabling LTE which is not desirable)

If any of those work, then check this file:

system\customize\ACC\default.xml

Scroll to the "Phone" app section

Look for a line "<item type="boolean" name="voLteEnabled">false</item>"

Change that to "true"

(If ARHD is compatible with AT&T, then it's probably already set to true)

If it was set to false, reboot and see if that works (to test, you'll have to undo any changes made in steps 1,2 or 3).

I did a lot of stuff before I did any of the above and Insert Coin (the custom ROM I use) already had the voLte setting set to true so the stuff below might actually be what fixed my issue. The next steps are just not as quick to accomplish as I ran the AT&T Ruu a few times and also flashed the developer ROM.

here's everything that i did...

Ran ATT 1.32.502.31 RUU to confirm that I can receive calls on 100% stock.

Flashed stock developer RUU and I could no longer receive calls.

Flashed custom rom (Insert Coin) and I could still not receive calls.

Ran ATT RUU again and calls suddenly worked again.

Re-flashed developer RUU.

Rooted it

installed root explorer (ES Files Explorer)

navigated to system\customize\ACC\default.xml and made the change explained above

rebooted the phone and I was again able to receive calls on the developer edition

Restored Insert Coin from a backup and i was STILL able to receive calls.

After restoring IC, i checked the default.xml file and noticed that not only was voLte set to true, ALL of the voLte options (there are like 5 of them) were set to true. Then I checked the IC zip file and opened the default.xml from what i was flashing and they were all set to true in the zip file. So I think they were all set to true when I began having problems. So I'm really not sure what fixed this for me. But, like I said, that's everything I did and it's working again.

If you go 100% stock by flashing the AT&T RUU (or if you're able to restore to stock using an untouched system image from TWRP) and still can't receive calls, then get on with AT&T support and have them verify your provisioning.

Sorry if this is not very well organized, but I started having this issue like last Friday and worked on it all weekend and Monday and finally got it resolved Monday night...so needless to say...I tried a lot of different things, but what I've written above is what I believe fixed the issue.

I think the problem is that your area supports voLte but maybe the ROM doesn't. Notice when you make a call that your 4G/LTE icon changes to 4G only. Well, the same is supposed to happen when you receive a call, but for some reason, something isn't allowing the phone to switch to 4g only.