You're already doing it in some other cases, like the quest_name line ...
but anyway ...
set @phone=ISNULL((select phone from guest where account=@account and room=@room_code group by phone),'')
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Personal Motto: Why push the envelope when you can just open it?
If you follow the direction given HERE[/url] you'll likely increase the number and quality of responses you get to your question.
Jason L. Selburg