Telephone Time!
This is just an excuse to tell you all to buy the DVD set "Mr. Show: The Complete First and Second Season". Somewhere in there - second season, I think - is a sketch of Bob and David playing a couple of guys in a job interview. Buy the DVDs, find this sketch, turn on the commentary track. The funny, it'll change your life.
Barring all that, though, having Max walk around the house, picking up phones is pretty funny, too. He holds them up to his ear with a "Uuuhhhhaa?" A sound which is the baby equivalant of "Hello" and/or a giant question mark Once he's established who the imaginary party is attempting to reach, hands the phone to either Mom or Dad.