I have 4 Pixels (two 8a, two 6a) all running on US Mobile (using T-Mobile's network). RCS stopped working on 2 of the phones randomly last week. The other 2 have remain connected, without issue. Setup is pretty much identical across all 4.
The solution that worked for me on the 2 bad phones was as follows (following @DirtyDan's process that has been popular and successful for many)
- Wipe all data and completely remove Messages.
- Ensure phone number verification was enabled (Settings -> Apps -> Sandboxed Google Play -> Google Settings -> All Services -> Phone number verification)
- Reboot the phone.
- Validate myaccount.google.com/phone was showing a validated Pixel device.
- Install Messages via Aurora, installing versionCode 242499063.
- Set Messages as default SMS app. Force stop and clear data if it loaded, and then give all permissions except location (which setting it as the default should have already done).
- Launch. RCS connected almost immediately in both cases.
- Updated Messages to current (20250808_03_RC04, version code 286373063), everything still worked.
I did have to reboot both phones again when RCS was still showing as connected but messages weren't flowing. Since then (knock on wood) things have been fine, including after updating Graphene from the 0906 to 0908 release,
Note that I did not have to give Carrier Services or Play Services any special permissions (they both had Call Logs, Phone, and SMS, as well as others), nor did I have to add the READ_DEVICE_IDENTIFIER permission.
My gut tells me that all of the RCS updates on the infrastructure side have not been accounted for in the Messages app (or vice-versa) and the older app's initial provisioning process is bypassing some or all of these changes. I don't think this is a Graphene issue either, as there are widespread reports of RCS issues across stock devices from multiple manufacturers on multiple wireless providers.
I also played around on my phone with the Messages debug menu (search, enter xyzzy and hit enter), specifically the RCS subsection. The "Report RCS Connection State" entry would crash the app when not connected, but now reports data successfully. None of the other submenus for RCS debugging provided anything useful.
Sorry for the length, but hopefully this is helpful to someone else.