When a fellow employee ignores your calls, that clearly calls for action. However, before focusing your attention on this unwilling receiver, the first place to look is at yourself.
If you tend to call this individual frequently and hold lengthy conversations on matters that you could and should resolve yourself, perhaps she is trying to send you a message. That does not excuse her failure to answer your call, but it might explain it. And if you do not call her back after being ignored, then her strategy has worked and she is likely to repeat it.
However, if you can honestly say that your conversations with her are businesslike, terse, and necessary, the next time she dodges your call, you should head straight for her office or workstation. And further, that is what you should do every time she avoids your calls. As soon as she realizes that she is going to spend more time dealing with you in person than over the phone, she will be more likely to take your calls.
You should also consider a more direct approach by simply asking her why she does not take your calls. You want her to answer the phone, and these steps should give you the answers you seek.
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