Firstly, the Orcon UFB service requires the MAC address of the connected device to be that associated with the modem they rented to you. So connecting the UFB service requires configuring the Fritz device to use this MAC address.
Log into the Fritz > Internet > Account Information and fill out like so, making sure to put in the MAC address from the supplied router:
For the VoIP settings, head to Telephony > Telephone Numbers > Line Settings Tab.
Fill out the location info then head down to voice packets.
The settings shown here work for me - https://imgur.com/LC9J1xQ
Your username will be your number with no leading zero, e.g. 39741120
Your password is not the same as your UFB/account password; I found mine by emailing the help desk and asking for my SIP username and password.
Head to the telephone numbers tab > new telephone number.
Put in internet number and user name as previously, e.g. 39741120, and your SIP password as well.
Put sip1.orcon.net.nz as the registrar, leave STUN server blank (not sure if Orcon has a STUN server or if this is needed). Hit OK and jump through the hoops. After being added you can edit the numbers settings although I'm not sure if there is anything else to configure here.
Lastly head to Telephony > Telephony Devices > Configure New Device.
Telephone > Next, FON1 > Next, Intercept all and so on (should be straightforward).
Wallah! Seemed to work fine for me after this process with both ingoing and outgoing working. There are surely some other little tweaks that might be useful but this got me up and running.