My wife mentioned that our car did this for a brief period of time whilst I was away this summer... but, it hasn't done it since I have been back. The subject has been mentioned once or twice before here on the forum.
When you take your Phaeton in for service, there are two 'Service Bulletins' (VW Campaigns) that your VW dealer needs to look at. These are Campaign 97J9, dated 03 February 2005, and Campaign 66C4, dated 02 June 2006. These are not recalls, they are what you could, I guess, call "Quality Assurance Update" programs for Phaetons. The second of the two documents, 66C4, makes a reference in Criteria 6 to checking the software version of the instrument cluster and updating it if necessary.
Perhaps wait until the car goes in for service and has this update done, and see if that eliminates the problem.
Neither one of these two campaigns are applicable to NAR (North American Region) Phaetons. In NAR, VW of America sends out Technical Bulletins on a case by case basis, and those TBs have already been discussed here on the forum at this thread: Phaeton Technical Bulletin (TB) Summary. None of these NAR TBs are 'recalls' - they are just guidance documents that elaborate on how to identify and resolve certain problems that may come up.
With respect to the two European Campaigns mentioned above (97J9 and 66C4), my understanding is that the VW dealers will automatically carry out any appropriate actions whenever the Phaeton next comes in for service. Not all the actions in each document are applicable to every car - it depends on both the VIN of the car (build date) and the equipment fitted to each car. It is also worth noting that the first of the two documents, 97J9, is only applicable to cars with a VIN of WVW ZZZ 3D Z 4 8 008 344 or lower - in other words, 2003 or early production 2004 Phaetons that were built prior to mid-September 2003.