Most likely it's the callsign of an AI aircraft. Are you using the default AI? I'm asking because the last communication with ATC before the fatal error is with "Orbit" which is one of the imaginary airlines. In the default traffic file, there is only one flightplan that both uses an Icelandic airport and has a UK (G) registration. I don't see anything unusual about it, so you are going to have contact the VOXATC developer at the official website.