My italk line was having issues with people ringing in and not getting through even though we were right next to the phone; the phone would just go to voicemail after about a minute of ringing.
There was also the problem with the Voicemail message waiting indicating sound not working which I was told by Slingshot that it is a known problem and will take 5-6 months to fix.

I tried ringing the customer service call center to try and fix the problem of people not being able to get a hold of us but the wait times were abysmal and then when I did get through to somebody I was disconnected. 

My solution:

Went to 2talks website http://www.2talk.co.nz and signed up for the 2talk10 which including gst is $11.50.
I signed up for a new number which is what I did with slingshot for the italk line in August only Slingshots iTalk took over a week to complete and there were various other problems with the setup.
2talks setup was complete within minutes and I had my Linksys SPA2102 up and running within 5 minutes after that.

People can get a hold of me now and I have voicemail waiting indication working; the voice quality is much better and I have way more features.

I have heard the 2talk customer service is pretty crap too but I didnt have to deal with them in the setup process and I set everything up including signup at about 11:30pm on friday.

So that I did not miss any calls from people ringing my old number still on italk (which is is a free account because I have naked DSL) I just setup a forwarder on my italk line to my new 2talk line which works great so I am not sure why people ringing my italk line could not get through to my phone when I just had italk.
Beats me :-)

Problem Solved.