What type of other sim are you using? It needs to be a Telstra or Vodafone sim, the device will work fine locked with Optus and Optus Resellers. Just to clarify, its not locked to the Sim its sold with, its Network Locked to only work on the Optus Network.
It won't work with an Aldi sim (Telstra reseller)which tested fine with Optus shop employee's phone but the phone did work fine with Optus employee's sim. No go with a Telstra pre paid sim or Amaysim. Optus support have kindly supplied the NUC (I am a customer with two other services) but this phone just won't give me the prompt to enter it.