Unfortunately there's not much you can do. If you are good with soldering you can remove the flash chip and wipe an area which stores the customization data.
There is a forum member which used to unlock these remotely for a small fee but he has been offline for half a year.--
Providers (through asterisk): voip.ms, freephoneline, smartcall.ro, ipcomms, callcentric. Hardware: Vonage VDV21, Moto VT2x42, Linksys SPA series, Grandstream HT series, Panasonic KX-TGP5x0