At an exchange level, this site gets the updates: Openreach "where & when"
On an individual phone line, the standard BT wholesale DSL checker is best, and applies to any ISP.
The phase numbering relates to the plans when BT announced the exchange would get fibre. Different exchanges move back-and-forth at different rates, but the phase number doesn't change.
For example, Woking was originally down for Mar 2011 (probably the same as Bath Kingsmead, and the source of Plusnet's map data), but recently got moved onto the "coming later" list as Dec 2011. It is still marked as phase 5b largely, with some cabinets delayed into 6a.