Im thinking of purchasing a unifi switch and router and wondering if I will be able to still have my VoIP phones working if the original router supplied by optus is plugged into the unifi switch with its gateway set to the unifi router. I've seen similar posts on this forum but the optus router sits behind the byo router inbetween the NTU. I've made a very basic Visio diagram to try an make my idea clearer.
Reference Post: https://yescrowd.optus.com.au/t5/NBN/BYO-Voip-Router/td-p/533071
What you propose would work just fine, make sure you forward port 5060 at the IP assigned to the Optus router for your phone service to work correctly.
Optus doesn't supply the SIP details due to support boundaries, there is way too many ways to skin a cat when it comes to VoIP setups... and it's impossible to support every single outcome (especially for scripted support).