The trouble with referring to adsl_phy.bin as the DSL firmware is it gets confusing when also thinking about the firmware image for the whole device.
Normally, if you wanted to try using a different adsl_phy.bin file, you would need to create a new firmware image for the whole device, the BB2 in your case, and upload that. If it's possible to build a whole firmware image, replacing the adsl_phy.bin file inside it would be a relatively minor change. Some devices will do checks on a firmware image you upload, if the checks are as sophisticated as validating a digital signature, then it wouldn't accept any custom built firmware image. Usually most devices are fairly locked down and won't accept a custom made firmware.
Sometimes, with full telnet shell access to a device, you might be able to transfer the adsl_phy.bin file onto the device by tftp or on a USB stick, and then command it somehow to load that adsl_phy.bin file from it's new location.
