Buffy at a Wizarding trial

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I'm looking for a story I read a couple weeks ago and apparently failed to save or bookmark.  Buffy (and Dawn?) have to go to (Harry Potter) Wizard headquarters on Council business.  I remember them going down underground in a phone booth. (Is that normal for HP?  I don't really know never getting through even the first movie.)  Anyway, the person she needs to see (Dumbledore?) is in the middle of a trial (Harry's?) and can't take time for her at the moment.  She meets several people there and they take up upstairs in the court gallery to watch.

I thought this was one of a series of different crossovers where Buffy's is collecting old Council debts, one of them being with Denny Crane of Boston Legal.  Found the Boston Legal story, by justaguy, but no HP crossover there.  Can anybody point me to the right story?