Not really much you can do with this. You would typically need to have a separate public IP for each unit. One thing is to purchase a call relay, put this behind your nat with the one public IP assigned to it, then have it “relay” the calls to the 2 A800’s on the private IP.
The other question is were you will have enough bandwith.
DSL with dinamic I.P. services do not have tipically that much bandwith on the direction from the user to the NET.
Yu will need around 256 kb of real bandwith on that direction, and that is hard to get on a normal DSL line.