If you visit the Netgear page on the Sipgate site at www.sipgate.co.uk/netgear there is an option on the right of the screen to "open firmware"

This will download and install the unlocked firmware, but be aware that (a) you will lose all your current settings and have to re-enter them (assuming you want to carry on using Sipgate) and (b) you will no longer be eligible for tech support from Sipgate in relation to the TA612V.

Ivor