mztykal,
Sorry for you woes.
Having been in management positions for most of my life, I always get around these problems by having one rule for everyone and every new employee gets it straight from me:
If you call in sick, if you are stuck somewhere, if you need to switch shifts, if you need vacation... etc... and you don't speak directly to me then as far as I'm concerned you just didn't show up. I know that this seems harsh, but implementing this has saved me two huge headaches over the years. One is cases like yours where two employees aren't actually on the same sheet of music; under my system, that woman would have had to tell ME that she was willing to cover for you and then SHE would be responsible for being there. The second case is where employee "A" calls and employee "B" answers the phone but forgets to give anyone the message. In either case, there is bad blood between the employees and though I don't shy away from it, I'd rather not have to wade into a fight between two people. This system also protects the employee... they know I'm honest and that if they call and I ok something, they are covered... no matter what.
I guess that would be my advice to everyone... if you are calling in or have some change from the norm, don't talk to anyone but the person who has ultimate authority... CYA is often a good thing.
Joe