BtVS/Angel by Whedon and Mutant Enemy. Hmm, Angel Season 3's 'Sleep Tight' aired in March 2002 and Buffy Season 6's 'Grave' in May, so the timeline may be a trifle skewed... Oh well, at least both scenes happen at night. Spoilers ahead. Taking a cue from Hotpoint and adding info blocks at the ends of these chapters.
"You've called the Magic Box after hours. Willow speaking."
"Thank God! I tried calling Buffy but no-one picked up at her house."
"She's back in the training room catching up with Giles. Want me to-"
"No, this is Fred, from L.A. We're all in trouble down here and we need someone powerful with the mojo."
"Angel has a baby boy, Wesley stole him to give to this hunter from last century and there's this time/space demon who's claiming responsibility for everything."
"Whoa and they say I'm mighty with the babble
. When did this happen?"
"The hunter guy just left here, he didn't have Connor, so Wesley hasn't handed him over yet. We need help now!"
"Time/space demon you say? Sounds like a really powerful spirit practically bursting with energy. I've only teleported once or twice before, so I don't know if I can get to you immediately. I'll try doing it in short hops."
"Thanks! We're still at the Hyperion."
"No worries. I'll be down as soon as I can."
Fred put down the phone and whirled around to hug Gunn.
"She's going to be here," she cried into his chest. "We're all going to be okay."
Still laughing, Buffy and Giles strode into the main room of the Magic Box.
"Hey, Anya, thanks for getting the phone," Buffy said. "Who was it-"
The Slayer froze in place, having realized that Anya was unconscious and being held off the floor by the neck.
"Willow!" Giles warned, readying a spell behind his back. "Put her down immediately."
"Sure, why not? It's not like I can absorb her ability to bend time and space. She just taps into it and the people higher up the chain from her can cut off the flow. Me? I'm hungry."
Anya fell to the floor as Willow rose off the ground. Cursing, Giles released a green fireball, but it was too late to aim properly as the black-haired witch was moving too fast. Before he could do anything else, she'd burst through the ceiling, leaving a giant gaping hole in her wake.
"Giles," yelled Buffy as she grabbed her Watcher by the shoulders. "What are we going to do?"
"I have a contact who can track phone records, so we should be able to find out who called with little effort. I just hope we can warn them in time."
This is the point where Willow, following her girlfriend's death, has gone on a dark magic binge. Giles contained her, temporarily, until Anya succumbed to her magically boosted telepathy and released her.