VFX web site shows that calls to USA mobiles are 5c a min, the same as calls to USA landlines. Seems to good to be true, what with NZ mobiles being 22c. Rates are at http://wordpress.xnet.co.nz/phone/voip-phone-line-vfx ,ratesheet link on left.
Just a brief rundown on the US numbering scheme, because you're obviously unaware. Calls to mobile phones in the US will always cost the same as a call to a landline, no matter who you use for your calling.
Mobile phones are allocated local phone numbers and don't have their own prefix like most countries.
The US has MPP (mobile party pays) for mobile calls unlike most countries that have CPP (calling party pays). Calling a mobile costs the same as calling any other number, and the mobile phone user pays to answer this call. In the old days this was typically a per minute rate, now it's typically a flat rate per month change or deducted from your monthly minutes.
*Need help configuring your Linksys ATA or IP Phones for New Zealand? Check my blog post