They couldn't hear the conversation at the moment but it will certainly be played ad nauseum by Skyler's defense attorney (Saul?!?) during her assuredly high-profile impending prosecution, at which time Marie will hear it.

Right. He knew Marie was there, and he knew how badly his actions ruined her and Skyler's sisterhood. In a desperate act to repair the family he shattered Marie must have been on his mind.

Walt's phone call victimized Skyler in front of the police AND Marie, allowing Marie to forgive Skyler's (in)actions as acts of a scared, battered women. The sisters can now have a relationship that wouldn't be possible if not for the phone call.