As long as the QE2 is still registered in Port Vila, then her call sign is officially 'YJVW6', as it has been since 2009.
The 'GBTT' was originally the call sign for the Queen Mary, and was then transferred to QE2. QE2 relinquished this call sign in 2009 when her port of registry was changed to Port Vila. When Queen Elizabeth (2010) was launched she took up the 'GBTT' call sign again, but had to relinquish it when her port of registry was changed to Hamilton, Bermuda, whereupon she became 'ZCEF2'.
QE2 could only officially take up the 'GBTT' call sign again if she was re-registered in the UK.
Had a chuckle when I saw the Marine Traffic website on 29.09.2015 - it showed QE2 doing 53.4 knots towards Odessa again, but only briefly.
Skilly