Why does my mother whine like a child to me, but the moment my husband walks in or her phone rings she stops whining and is chipper?

I would ask her if she would like some bread and cheese with her whine.

I find it annoying beyond tolerance when a grown human behaves like a spoiled child.

You can stop this by ignoring her when she does it. Only engage with her when she is talking appropriately. Tell her that you will not speak with her until she stops, she will probably throw a few tantrums until she figures out that you are serious and you will not be dealing with it any longer.

I would also record her so you have evidence of her behavior.

She probably shouldn't be trusted with any issues between your husband and you if she is always taking his side. You never know what she is saying behind your back.

This behavior is a form of manipulation and should not be entertained.

It sounds as though your mother is well aware of what she is doing, and can choose how she acts. If the whining is too annoying, too constant, just walk out and say ‘I’ll come back when you’re in a better mood’. You need to give her some incentive to be pleasant to you too.