Personally, I wouldn't go with Unlimited Internet - they're quite dodgy (See this thread).
Voyager or 2degrees will give you SIP details if you ask nicely. Else don't forget you could always port the phone to 2talk and use that if it is a must. Spark are expensive but good and include added bonuses like Netflix and Lightbox too.
I've been with Unlimited exactly a year today
In that year, I've never had to call them for anything, I can't ask for any more
They were also the only ISP that connected VDSL to my house, where all others said it couldn't be done, ADSL or nothing
And here I am a year later with this mornings 6db profile change sitting on 72/21Mbps connection
I moved from 2Degrees who refused to do any work on SIP login/PW on non supplied routers
One of my FB7390's (from Snap) died, I needed to setup my spare, was a mammoth battle I lost, so I moved to more accommodating ISP
Not looked back since.
Anywho, the FB7390 sits happily behind my ERL3, it does nothing but VOIP
I like the FB as its a DECT station don't need any BT plug points, the FB covers the house with handsets