I bought a carrier-unlocked Google Pixel 6a from Google's online store and had it delivered to my home in the US. I put my AT&T SIM card in it. I was able to make and receive calls normally at first, but one day, I noticed that I could no longer do this. I had both a cellular connection and a Wi-Fi connection with Wi-Fi calling enabled. I read this part of the GrapheneOS guide to find a solution. I first tried installing Google's official phone app, but this did not work. I then tried resetting the mobile network. After my phone had finished rebooting, I was able to make and receive phone calls again. Then I put my mobile network settings back to where they were originally. This included connecting to my Wi-Fi and setting up Proton VPN again. I was still able to make and receive phone calls, so I thought that my problem was solved. Now, after two days, I noticed that I am unable to make or receive phone calls again. All calls go straight to voicemail. I have no idea why it stopped working.
I should also note that I have been able to send texts through SMS and use mobile data without any problem, so I do not think that there is an issue with my phone plan. Please let me know if there is any other information I should include. Thank you.