Woah...
First of all, the iPhone doesn't use any "special sim" card all SIM cards work but if you want Visual Voicemail to work you need an iPhone tariff. The iPhone tariff adds access points to your account (not SIM) so it works, if you happy with just using the 901 Voicemail service you don't need to add it.
If your wanting to use a pre pay sim with all the features it will cost you £10 a month for unlimited access to the BT and Cloud WiFi network as well as the mobile network and you get the Visual Voicemail access.
fireflybob,
Yes it's true.
There are also facilities that are blocked on the "standard" iPhone, for reasons I've never quite understood.
Such as?
If you have any questions feel free to fire them to me, I work with O2 and Apple on the iPhone.