Final step in my carrier migration: the visual voicemail (VVM). I was quite surprised that it didn’t work out of the box. The hotline confirmed that VVM was activated on the carrier’s side and that I should try to synchronise with iTunes (to be sure to have the latest carrier settings) - but that didn’t help.
The hotline engineers told me then that my iPhone needs to marked as unlocked in Apple’s device database. But even after the device database was updated, it didn’t work. Then the Swisscom engineers changed one more configuration setting and asked me to update to the latest firmware (3.1.3). The configuration change or the update (I don’t know which one) did the trick:
The conclusion: it may not be enough to unlock the iPhone with Ultrasn0w to change the carrier and have a working VVM, maybe it’s necessary to acquire an official unlock.