Disclaimer: Numb3rs belongs to CBS and the Scott brothers and many other people who aren't me.
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Colby. The voice is raw and ragged on the other end—which makes sense since Colby's obviously been running—and the background is filled with noise. Train noise. Colby's on a train or near a train. If the call was being recorded, then technicians would be able to play it back and determine if it was a subway or surface rail. That's beside the point--Colby is calling her. Why? What could he possibly expect to get from her after betraying everything so thoroughly? Maybe he's expecting Charlie to let feelings and memories cloud her judgment. If that's the case, then he's going to be sorry.
"I need to talk to Don."
Six words, and Charlie feels deflated. Of course. Don. In the end, it's always about Don. No matter how hard her brother protests that Charlie steals all the attention, when it really matters to her, the attention is always focused on Don. "Then why call me?" She tries for vicious but knows Colby will be able to hear the whine in her voice. She'll never escape being Don's whiny kid sister, just like she told Colby one night when they were full of shrimp-fried rice and too comfortable to move.
"Because the FBI doesn't have enough time to trace me through your phone, Charlie. Think like a mathematician. Is there any reason not to put me through to Don?"
There's a split second where she contemplates just shutting the phone and going back to her algorithms. Just out of spite. And Colby knows her so well that he calls her on it before she's even fully processed the thought. 'Think like a mathematician'--fine, if that's how he wants her thinking, then she'll oblige. Her thinking like a mathematician is going to put him right back behind bars.