This has now officially launched
http://www.xfonemobile.co.uk/home.aspx
Xfone/Swiftnet have been previously selling the IoM SIMs from their UK-based Auracall and Story brands, plus a reseller of theirs called Simply-Fone. They have a range of other services including several calling card and other callthrough arrangements, so they aren't beginners.
This is an actual UK mobile number, in a range allocated to a wi-fi and other non-standard providers though, so some international providers with a range of tariffs will charge more eg Voicestick, while most will be the same for all +447n (n not 0) numbers. For British users, it will be interesting to find out if calls to these 078221 numbers come out of contract inclusive minutes, and whether such calls can be forwarded to the SIM.
I'm browsing through slowly to compare rates. There are plenty of countries with free incoming calls, though it sometimes varies on different roamed networks within a country.
There's a phone (I don't recognise, but quad-band) included on an offer for the first 100 customers, £39.99 with £25 credit, and I have email details of this which I could forward.