Be sure to look at my answer to "orange females" as it relates to your question.
Tabby brings in yet another set of genes (more, in fact). Your calico should show what kind of tabby she is on her orange patches, if they are large enough. She is an orange/nonorange white-spotted tabby in genetic terms!!
The orange/nonorange is a tortie. Now, if you think about it, a female has twice as many X chromosome genes as a male (the Y is tiny and carries many fewer genes). This imbalance is solved by cells randomly switching off one of those Xs in a female early in development. So it's possible for a tabbie to be, e.g., brown, but have the tortie color genetically if most cells had the orange gene switched off during that random process.
I'm more concerned about where the white spotting came from. That gene is also codominant, so a cat with one dose has some spotting, a cat with 2 doses has a lot more white spots. Does either mom or the suspected dad have white spots? One of them must have had white spots to produce a calico kitten.