Since I had to look all over�for the correct settings and there was a
lot of trial and error involved (specially for the Realm). Thanks to Engin Support that finally provided this info after logging a support request.

The screenshots are from a Nokia E65, but should be applicable for similar Nokia phones.

General

Profile name: engin
Service profile: IETF
Default access point: {Your WLAN Access Point}
Public user name: sip:{phone number}@voice.mibroadband.com.au
User compression: No
Registration: When needed
Use security: No

Since I had to do some research on this topic recently, here are the results that I have found. These plans are interesting for people that do not call out a lot, but rather use the phone to be accessible.

GoMobile – Pay as you go plans

Southern Cross Telecom – Pay as you go plans

AAPT – GO15 plan (**needs to be bundled with Landline phone)

** – some conditions apply to qualify for $0 minimum spend)