Before BT launched Broadband Talk it was called Broadband Voice and used a different platform to connect the calls. I don't remember the platform name but any member of the back office team should be able to tell you.
The SIP protocol is the method that BT have now moved to for VoIP and is pretty much the industry standard for delivering such VoIP services.
(The easiest way to look at it, or at least explain it, is as an Operating system I guess)
They ran Broadband Voice for around 18 months on the old platform and have been using the SIP platform now for around 18 months giving them 3 years in the VoIP market.
To standardise their network, all of the old Broadband Voice customers on their original transport have been moved onto the SIP platform, they obviously did not do enough research or invest enough in server capability before doing this or something along those lines and the increased volume of calls, users and/or numbers caused the server(s) BT use to run this service to collapse.
Regarding your 05 number, you should be aware that there are people on certain telephone networks who will be unable to call that number. Also, people calling that number will be charged for calling you as not even BT include 05 numbers in their inclusive calls.
I don't know how accurate this is but some of my friends have reported that they have been charged over 30p/min from their mobile to call it, so you might want to warn your friends to check the cost of calling it before you give them that number.
As I said in a previous post, a member of the Broadband Back Office team have advised me that the only way to get a geographic number for Broadband Talk is to cancel the service, for which there is a penalty fee payable if you are within the 12 month contract, and then re-sign up for another year.