could well be a voip form of line, if i remember correctly bt's voip broadband scheme (Broadband Voice) was using similar number allocation to this one.
http://www.bt.com/broadbandvoicepackages/faq.html#6.1
<quote>
Can I request a specific telephone number or ask for my number to be changed?
Unfortunately, this functionality is currently not available. The Broadband Voice numbers which start with 05 are automatically allocated when you order the service and cannot be changed.
</quote>
p