Hi Beth - thanks for responding.
So, signal in my area is very good with full coverage up to 4G.
The issue is not specific to a location as far as I can tell. All other services are fine - data/text is unaffected.
As an example, today, the mobile is working - I can make and receive calls. I always know when it will work as when I dial, my phone screen always says 'conditional call forwarding active' almost immediately. Why it is working today, I have no idea - tomorrow, it very well may not. Even on the days when the issue is prevalent, I might (emphasis on might) be able to get a single call out after many attempts - I've even tried re-registering on the network but that doesn't really help. This has happened in the distant past, but for a very short period (day or 2) - this time it has been 3 weeks or so.
I didn't know until this week that my partners phone was doing the same (she very rarely uses her phone to actually call people so she was unaware) - I tried to call her mobile but couldn't so then tried her mobile to call mine plus a different number to rule out the network, but her mobile didn't dial out to either number also - that's when I thought it must be a network/SIM/Account issue (we are both on Virgin and the SIMS are managed on the same account).
I know that because there is no structure to the call send/receive failure it might be a conundrum, but your further help is appreciated as I am missing lots of calls and we are in the process of purchasing a new home so the calls are important.