poor call quality iphone 12 mini

My call quality has never been great, even before switching from Verizon to Visible, but lately it has been awful.  Specifically, there are very frequent lapses during a call. There is no discernible pattern to this, it can happen when I am on cellular data with four bars, and it can happen when I am on my home wifi (which is consistently above 200 Mbps) with no competing devices on the wifi. In addition to lapses, dropped calls are common, and there are times when I cannot make a call or send a text without restarting the phone.  

Things I have already tried (repeatedly):

• restarting the phone
• resetting network settings
• verfifying that wifi calling is enabled
• updating iOS

I am currently experimenting with not using the protective case on the phone, though I am not optimistic about this since it is a pretty minimalist case to begin with.

I am running iOS 17.1.1 on on iPhone 12 mini. I'd be very grateful for any tips on how to pinpoint whether the problem is with my hardware, software or with Visible.