ISSUE: Can't place/receive calls.
Incoming calls go directly to voice mail, but no message notifications are received. Outgoing calls seem to connect and then the call ended screen is shown. The issue is particularly frustrating as the network identification name is still visible at the top of the screen so there is no way to know the phone isn't working until trying to make a call or getting a text from someone asking why you aren't answering your phone or responding to voice messages.

Text messaging is unaffected.

TEMP FIX: Reset mobile network (Settings > System > Reset options > Reset Mobile Network Settings).
The first time I experienced this issue I was travelling and my carrier tech support had me reset the mobile network which resolved the issue. I didn't experience the same problem for perhaps a year. Now resetting the mobile network only provides a temporary fix, usually one call, occasionally more.

For a while I was asked to confirm each network reset request with the message, "Reset your internet? This will end your phone call." I haven't been on a phone call when seeing this so I thought it may be a clue, but I stopped seeing this confirmation request and the problem hasn't gone away.

I've been pouring over the phone's system and phone app logs for clues as to what might be happening, but I'm not a developer and I haven't found anything obvious. The system logs show the disconnect reason=36, as far as I can tell this is a generic error code. The problem didn't seem to be associated with an update or change in settings. I've installed several updates since this problem became chronic.

Device detail and full list of troubleshooting steps I've taken are below. I'm at a loss. Any advice/next steps?

DEVICE
Pixel 6a (purchased new and unlocked from Google),
Hardware SKU-GX7AS,
Hardware Ver.-MP1.0

TROUBLESHOOTING STEPS⸻RESULT
-Toggle Airplane Mode⸻No change
-Reboot phone⸻No change
-Check SIM card. Eject & reinstall⸻No change
-Update Software⸻No change, multiple updates installed since issue became chronic.
-Change Preferred network type (Settings > Network & internet > SIMs > Carrier SIM > Preferred network type: 5G, LTE, 3G, LTE only, 5G or LTE, or 5G only)⸻No change regardless of which type selected
-Toggle VoLTE⸻No change. My phone is usually set to allow VoLTE.
-Allow 2G calls⸻No change
-Disable Wi-Fi Calling⸻Already disabled.
-Use default phone app⸻Done. I never replaced the default phone app.
-Check whether issue exists in a new user profile⸻Yes, problem exists in new profiles, and old user profile.
-Ensure Date and Time settings are correct and set to automatic⸻Date & Timezone are correct and auto update from network enabled.
-Contact Carrier Tech Support⸻New SIM issued.
After a lengthy call where I performed various tests and duplicated the near immediate return of the problem after resetting the phone's network settings the tech could not identify the cause so they sent me a new SIM.
-Replace SIM⸻No change.
-Contact Carrier Tech Support⸻No change.
After new SIM did not fix the issue the second tech told me the problem was that the phone was locked by a previous carrier and I needed to contact the original carrier to have the phone unlocked. The device has never been locked. I bought the phone new, directly from Google, without a carrier contract. The tech refused to believe the phone wasn't locked and told me there wasn't anything more he could do to help me. I saw another user had this experience, but their issue seemed to be resolved after a software update.
-Switch mobile carriers⸻No change, problem persists.