Sharing pairs between Ethernet and phone lines is possible, although not recommended. Ethernet uses pairs 2 and 3 (orange and green). You could install an adapter at each end of the cable, taking pair #1 to one jack for phone and pairs #2 and #3 to another for Ethernet.
There should be no significant impact on DSL signal, considering it's only up to 300 feet long run.
However, if the walls are not closed yet, seriously consider adding a separate cable for Ethernet. At construction stage the expense of putting additional cables really pays off with added flexibility and decreased amount of problems with wrong pin layouts, adapters and so on. Also consider that few years down the line there may be applications introduced (like Gigabit Ethernet) to the home market that would require all four pairs to operate. When that happens, you'll have to run additional cable.
Dmitri Abaimov, RCDD