willnz: Hi Ray,
I do currently have my Linksys ATA connected to my asterisk box at the moment for VFX. And yes, Caller ID does work :)
I was also considering a GSM gateway and TalkZoneZero - but GSM gateways are pretty costly (even for a single port one).
Yes, you can get a VFX line in NZ and use it overseas. You just have to have a Napier address for a Napier number.
Will
Something to remember with GSM gateways is it is possible to get a rebate towards the gateway. Vodafone looks very favourably on GSM gateways.
For example (I know this isn't applicable to you) but a company who gets a gateway, plus possibly swaps a couple of mobiles from Telecom to Vodafone, plus a couple of resigns, and the rebate could be worthwhile. I can't give you more definite answers as it's such a sliding scale.
If you need more help, I can give you the details of the brilliant Vodafone rep that we use for all our deals. PM me for more info.