Hi @rik,
Thanks for your reply. I saw related issues in this and this post.
My carrier is Chunghwa Telecom (the largest carrier in Taiwan). Taiwan shut down 3G in 2024, so VoLTE is required for any calls. Not sure how I could get a call on the second SIM. The second SIM is with congstar (Germany) but I guess in that matter it would be more important who is congstar’s roaming contractor.
I visited my carrier in person. They tested by putting my SIM into one of their own phones where it worked fine, confirming the SIM itself is not the issue. They concluded the problem is on my phone’s side.
Regarding timing: I updated from 7.3 to 7.4 around April 5th. IMS seems to have been working on 7.3 (I have a missed call on 1st April). The problem may therefore have been introduced in 7.4, not 7.5 — I only updated to 7.5 later (at the store) and it did not resolve the issue.
I currently have a workaround (using a FP3 running on LineageOS to receive and do calls + using a hotspot) and am busy until the weekend, but I am comfortable with adb and happy to run diagnostics then if that would help. Please let me know what output would be most useful.
Best,
yijiake