Dear Lady,
I guess you've become a 'pet client' of mine because I've been so 'helpful' in the past. However, my day and work processing does not stop for you. Sorry that I was on the phone or in meetings when you called me and I was unable to call you back in 'your timeframe'. Leaving me a rude VM is not going to help your case or encourage me to offer you additional assistance/compensation. I'm willing to be more helpful if you are nice to me.
Kind Regards,
The person that can bend the rules if I want to, or not.
PS Not being available when I call you back is not helping your case since it's so 'urgent' that you have to be rude.