apple watch 3 cellular voicemail access

I've just got a new Apple Series 3 Apple Watch (GPS, Cellular model) and when I access the voicemail tab (under the Phone app) all I get is "Access Voicemail on iPhone".

My carrier (Optus) says that this option isn't supported by Apple or Optus. Is this true? If so why bother having a voicemail tab at all and force me to just dial the access # instead?