Difficult conversations with the boss?

This is why Batman works alone.  Get another job and free yourself from the ball-and-chain.

Time for another  job .  Being scrutinized that way is demining.. 

Don’t let it bother you and don’t make promises you cannot keep.

She’s a bad manager: her style is to simply bully you all by cracking the whip whilst crying “More!”. Even if you were outstanding she’d still be making the same demands; probably worse ones as like all such poor managers she’d be worried that you might take her job.

By all means make clear to her that you’re very aware of the need to be productive and efficient, and that you’re constantly reviewing yourself to see whether you could do better. If she wants to still play the same old tune that you’ve repeatedly heard then ask her what she thinks could be done. Ask her to demonstrate her methods, as that would be instructive to all of her staff. 

Be a bit passive aggressive about it. If you kick back just enough to make her job more unpleasant but not enough to merit any kind of reprimands then those kinds of managers usually find someone else to pick on. If you can, get to know *her* manager. That can work wonders.

Also share what’s discussed with your co-workers. You may well find proof that manager has been playing the same lazy tune with your co-workers. Just knowing you’re not alone can be an enormous stress relief.

Have clear, detailed actions you are going to take to improve performance based the the metric used to measure your performance.  It is great that you have a positive, "can do" attitude, but what matters to the boss is whether or not you are actually able to improve performance.  If you've promised to improve in the past and haven't, why?  

Well, has your boss heard it all before? By that I mean if these are regular conversations and the content is pretty much always the same in terms of the performance gaps and opportunities for improvement, if you (and your coworkers) always give the same lip service responses to your desire to improve but that isn’t supported by an improved performance, well, it’s no wonder your boss is cynical. You would be better served by being able to give specific examples of what you have done to make improvements since the last feedback discussion and your specific plan of action for how you intend to improve your performance going forward. To simply say “I plan to do better” without specifics is an empty statement and will not make a positive impression on the boss.

Call centers are often toxic environments by nature.  That boss's job is to crack the whip so to speak to keep the employees taking calls as fast as they are able to.  So that boss is doing exactly what she is paid to do by getting on people's case when the number of calls they take drops below whatever metric someone decided on.

So look at your boss as a robot giving a pre-recorded message.  Nothing you can say will alter the contents of that message.  Just endure it until it is over and do the best job you can.

Your problem is you presume its "negative". Why?

Talk about the differences you've seen in the past weeks that varied from the last talks you did, things shift and they need to understand any known variation showing up now.

You can't change your bosses poor management style.... and tell her "I appreciate the feedback in a constructive way" and that some things are out of your control like "it depends on the caller and what they are after and how many emergency calls interrupt the answering of the calls." ....that you will continually try to improve, is all well and good but  you need to know "what are her constructive suggestions to do that"....... silence is a great way of dealing with a person like that, it is very difficult to deal with silence, but if you ask "what are her constructive suggestions to do that". and then remain silent she will be forced to fill that silence with an answer...and if she fills it with a question just ask the same question again and then silence

It's all well and good to say you will improve, but are you actually improving or just telling her what you think she wants to hear?  If you've promised improvement repeatedly but have not delivered, your words are hollow and she will get tired of hearing the same old thing. If you have a good explanation for the gaps, give it.  Instead of telling her you will improve, ask her specifically what you can do to improve.