The issue is very possibly related to the fact that Optus have apparently started shutting down 3G. They may not be shutting down every coverage area at the same time, which would explain why your calls were connecting one day, but not the next.
As you probably already know from extensive discussions in these forums, Samsungs lose VoLTE capability whenever a custom ROM is installed (thanks to Samsung’s closed, proprietary IMS implementation and the current inability to reverse engineer it). So yes, in one way you could say that it’s an OS problem, but likely not related to that /e/OS update specifically.
Of course, Optus’ customer service probably don’t know that you installed a custom ROM and lost VoLTE capability. All they see is that you’re presumed to be using a standard, VoLTE-enabled Samsung device. Or if they do know you replaced the OS, then the customer service rep might not be aware of the resulting loss of Samsung’s IMS.
That’s my assumption, anyway, supported by the fact that you get normal call functioning when moving the SIM to a different device (presumably either non-Samsung, or if Samsung, then one running standard Android). The fact that a different SIM has the same problem in your device also supports that assumption.