Sure, some people just don't like answering the phone anymore. For most other people, though, a ringing cell phone is just way too hard resist. Hell, no-one can resist a WhatsApp notification for more than seven seconds - imagine if it's a matter so "urgent" that it requires a phone call. Imagine what kinds of super juicy gossip could be waiting on the other end of the line.
No, there's only one real reason why she wouldn't take a phone call: because you're in the same room as her, and she doesn't want you to overhear what she's saying. Or who she's talking to. And the only reason that might be is she's organizing you a surprise birthday party, or she's organizing some hot action for herself on the side.