We've been having trouble with an 877 on a realtively long line on an ADSL2-enabled (21CN) exchange.
It works fine on other connections, just not this one!
Cisco, via our support provider, haven't been very helpful - they're not geared up to the "try this firmware to see if it fixes it" approach instewad expecting you to be able to get DSLAM and line card info to them so they can work to specifics. Getting that info from an ISP seems to be nigh on impossible (as they have to go back to BT for it)
IMO Cisco have really dropped the ball on this one - their kit just isn't as reliable on the ADSL side as equipment costing 1/10 the price and their support structure (for which you pay a significan amount) jsut can't handle mass-market "consumer-grade" services where you don't have easy access to the teclo's techincal team.
I'm glad I ony have the one 877 and have comissioned other sites with cisco kit for the LAN edge, but using cheap "consumer-grade" routers for the ADSL connection.