I have the same issue with my Iphone XR loosing signal between (Nov19-Feb2020) and dropping calls in the past 3-4 months whilst in lockdown and working from home.
I am being contacted by people by SMS or voicemail saying their calls went straight to voicemail. I raised this with Apple and I have been asked to contact O2 and giffgaff. Apple say the diagnostics are fine (?!).
Today Apple tried to call me 4 times and I had missed calls and voicemails. The phone was in my hands and I was able to take photos with the missed call notification and reception signal. Despite this Apple claims the phone has no issues.
I am now planning to write to a giffgaff agent to seek further advice.
Have you experienced similar issues and what did you do?