Unlike V.90, which is an ITU standard, DSL is not really standardized. The first part is the subscriber data. The first is CAP, which is found mostly in Cisco gear, and the second is DMT, which is found in gear from almost everyone else. While the CAP and DMT encapsulations are standardized, the DSL control protocols are not . Cisco uses a different control protocol than does Copper Mountain, which uses a different protocol than Lucent, which uses a different protocol than Fujitsu.
None are compatible.
This proprietary equipment is extremely annoying if the owner of the CPE happens to move across town. If you have DSL in US West territory and you move into GTE territory, then you can't take your Cisco 675 with you. With GTE to US West move, the same is true. Thus, the plan of the telcos with the CPE is clear. The cheapness of the CPE also allows the telcos to upgrade service and force obsolescence if they so choose.