Ok I've given this a go with some success.
Downloaded Fring from the App Store and filled in the registration information. Then configured the sip add-on as follows:
User Id - 1234567@sip.plus.net:5060 (where "1234567" is your 7 digit extension number)
Password - your password
Proxy - natproxy.plus.net:5082
It registered straight away and I was able to make outbound calls and receive incoming ones. My Son is well pleased - he thinks I've converted his iPod Touch to an iPhone

. I was only able to test voice one way as we don't have a microphone for the iPod yet but he intends to buy a Bluetooth headset.
There are some big limitations however, the most obvious one being that you can only use it in a Wi-Fi area. Also you can only accept incoming calls when the Fring application is running and it doesn't seem to support tone dialling as I wasn't able to access voicemail ("press 1 for new messages" etc.).
Still it's mighty clever and although it might be of limited practical use, I'm impressed.
Bob