Reading between the lines a bit, it sounds like you might be willing to tolerate the areacode being null. You can still catch invalid phone numbers if you were to push the isnull down one more level. Something like:
select IsNull( isnull(Area_code,'(000)')+Prefix_code+Last4_code,'Invalid Phone')
from clients
would only bounce this as invalid if either of the last two pieces are null.
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Your lack of planning does not constitute an emergency on my part...unless you're my manager...or a director and above...or a really loud-spoken end-user..All right - what was my emergency again?