I like you, tried all sorts of things including different sim slot until stumbling on the Flight Mode fix.
I also cleared the cache in the dialer app plus in apn settings reset to default. Did nothing at the time but maybe worth
My sim does not display my number either. I don't think its your sim as you are getting data.
I would report to customer service, form on web site. I have already but yet to get a reply that fix's the issue.
I will ask tonight on Volla's telegram channel.
If you can get your hands on a optus sim, all should work as optus 3G works with Volla ie 3G for calls, 4G for data.
For my own interest, I just turned my phone completely off, turn back on - no calls. Flight mode on/off - can make calls.
Maybe try in that order.