Calls not connecting (incoming and outgoing)

I joined iD Mobile in December 2020, and since then I have had lots of problems receiving and making calls.

When people try to call me the experience varies - sometimes the call doesn’t connect at all, sometimes it rings through and then connects to voicemail, sometimes it goes straight to voicemail.  The common thread is that I receive no notification of the call at all, until I am told that a message has been left.

When I try to call out, the phone tries to place the call, but just ends up switching to “Call Ended” without offering a ring tone or anything.

Neither of these problems is constant, but they do seem to be more often than not.  I have tried rebooting my phone and switching Airplane mode on and off.  My handset is one of the approved handsets, I have tried turning WiFi calling on and off to no avail.  Signal is good.  I’ve tried placing my SIM in another phone, and someone else’s phone in my SIM.  The other SIM in my phone worked, but so did my SIM in their phone - the fun of intermittent problems.  But honestly, if I can’t reliably receive phone calls, then I shall shortly be leaving the network.

Is there any way of debugging this or solving it?  Are other people suffering similarly, or is it just me?