Update.

Phone switched itself off after 30 minutes. Assume it automatically shuts down if frozen for too long. That's a relief.

Anyway, tried to synch contacts again ... again it freezes, though this time not fatally. Gets to the select contacts screen, but it's not interactive ... nothing is selectable or manipulanble.

I suspect it has problems with the number of contacts I have, and it runs out memory.

I have ~450 contacts (at least 800 numbers).

Edit: Phone will randomly reboot when trying to make this work.