I looked here for a guide to installing a Grandstream ATA I got from Go Wireless NZ, but couldn't find anything, so I got WorldxChange to 'open' our VFX line, filled in the values they gave me into the HT502's very basic UI, and it just worked! Excellent sound quality compared to my five year old PAP2T (as one would expect). Excellent price too.
Disclosure - Go WiFi gave me the ATA, as part of the fee for some VoIP training.
Interestingly, the email they sent, warning me for the zillionth time that I was losing support for my ATA configuration, suggested that this forum is an excellent source of support.