I highly doubt it is anything to do with adding branding. I have been in the IT industry specialising in VOIP since the late 90s. It is not a trivial matter to get something like VoWifi to work between 2 different vendors if either would be compromised by doing so. If Optus infrastructure does not support the Huawei implementation of VoWifi, this could be due to a non-conforming component in either system, or simply a mismatch. Optus can't simply change their network for one brand of handset, potentially at the detriment of others.
I am in the same boat here, with a .206 build that does not pass CTS. However, it is easy to be a keyboard warrior and cast dispersons on both Huawei and Optus, but it is probably a bit more involved than most people think. VoWifi is a complex thing to setup when it has to be totally seamless, hopping between it, VoLTE and 3G. The more complex the networks become, the more we will see these problems I would suggest. Optus has sold us a product that essentially worked fine since it was introduced. We can talk about security patches being out of date, etc, but unless there are security patches that affect us, or the devices, I think we are being impatient expecting updates every few weeks. I accept that I have mucked around with my handset and now I am living with the outcome.
Just my 2c.
