Voxalot have launched their own Facebook-based telephony applet called VoxCall, which allows you to have a "call-me" button on your Facebook profile. Going further, it's possible to use the button to call your other FB friends (assuming they have installed the applet too), and you can even set-up a conference using one of the conference rooms they have provided. Calls and conferences are established using VoIP and are free of charge, regardless of location of those taking part in the call.
Interestingly, the VoxCall applet is a pure SIP implementation that requires you identify you phone with a SIP URI (e.g. sip:fred456@voipuser.org), rather than a telephone number. A quick test here confirms that VoxCall works with
VoIP User, and also it works with
Truphone URIs, which means you can take Facebook calls on the move too. When you save a URI on the configuration page, Voxalot make a test call to your number/URI and then you confirm the connection by entering a 4 digit pincode they display on the FB screen.
Sydney-based Voxalot offer a range of telephony services to Internet Telephony Service Providers (ITSPs), but also support consumers directly with a range of VoIP packages, starting with a free model, VoxBasic, and going up to VoxExtreme at US $40 per month. A list of Voxalot's International ITSP partners can be found on their website.
Links:
Voxalot
VoxCall Wiki