Might just be worth an e-mail saying that "Since you've tried to contact her so many times over past XX days and been told she would ring back, but never does, you can only assume she is looking for alternative employment and her heart isn't in it at this organisation, so you will be contacting someone higher, once you've dealt with a few other calls."
It might shake her into accepting that she needs to respond, or you will be questioning her abilities directly with someone higher, and painting her in the worst possible light!
It gives her an hour or so, in case she really is at lunch, but then you have every right to ignore her and contact the organisation at a higher level. I suppose you could do one last call, and if she's 'busy' then ask for the extension and name of "someone more senior" ... Why should you waste your time (and money) chasing her any further.
If you are asked to 'hold' before getting a name and number, then it will be because whoever is taking the calls knows it is Trouble with a capital T if this avoidance routine continues.