Are you sure Swisscom is selling the iPhone unlocked? I believe that they are selling it with a prepaid option, which means that there is no ongoing contractual obligation and no monthly fee. However, it is often the case that phones sold this way are indeed vendor locked, which means you cannot just buy the phone and use it with whatever SIM card you like. Have you checked this explicitly? I tried to look into the Swisscom terms and conditions and it was not clear (it may be that you essentially need to registered to find out the detailed terms).