The hard way you have already discovered:
SX762 > Adsl Annex A > Analog line
SX763 > Adsl Annex B > ISDN line
You missed from the same source
ftp://dl.solcon.nl/pub/dsl/firmware/...SX762/Analoog/.
Unfortunately pretty old version 2.3.52.6.300.
You will hardly find a newer version because the "free" source is available only to ISP.
Try if you can find from an English operator, who typically use Adsl Annex A.