BT Phone service broken, fixed - but broadband unaffected!

[#1]

This morning I experienced extremely loud noise on my BT phone - sufficient to make it unusable. However, ADSL (same line) was unaffected - currently 7648 kbits.
Indeed, I reported the problem using the BT website.
Everything is all fixed now - but how can ADSL work whilst voice fails?
Indeed, if I had a VOIP phone, the outage would not have affected me!
The BT person thought it was probably "something to do with a battery connection". Do any of the GPO/BT ex-employees know what this means?

[#2]

ADSL uses a different frequency range to that of voice (hence the ADSL filters needed to connect a phone so it does not interfere with the ADSL signal).
The noise may have been limited to the voice frequencies and thus would not affect the ADSL signals.
While you may have remained in sync at the same speed, I suspect there may have been a lot of retransmits of data due to some of the noise bleeding into the ADSL frequency range. Having interleaving on the line could have minimised the disruption as well. It would have been interesting if you did a speed test.
Can't help with the battery Q.