I too have a cordless digital phone and the volume level of the phone is not as high as it used to be with my previous cordless phone. Previous one was also digital but not a DECT one, the current phone is.
Broadband should not have any affect on the quality of the phone line itself, although the signal between the handset and base station is digital it is changed back to analogue when leaving the base set to the phone socket in the wall.
My previous cordless phone was as clear and loud as a corded phone, but I have tried several of the newer DECT phones and none of them can produce a satisfactory volume level for my personal taste, even when set to the loudest setting for the earpiece. :o(