My employer recently offered to take over financial responsibility of my cell phone. Because of that switch, my wife's phone is the only device on our AT&T phone plan. I was talking things over with my father and he brought up the idea of moving her phone over to their plan, since they are also on AT&T. It doesn't seem like the phone company would like this since it would be costing them ~$50/month even though they are providing the same service. Is there a way I can do this without arousing suspicion, or do I even need to be covert in the first place? Note: She's eligible for an upgrade so we can cancel at any time, but she'd like to keep her current number.
My experience has been that as long as someone's willing to pay the bill, the phone companies don't care so much who that is. Your wife is your parents' daughter-in-law and therefore "family", however that may not meet the phone companies' technical definition of family, as she lives with you and not her parents. So, read the fine print; what qualifies as family and what doesn't is all laid out in the service agreements. Again, like I said, they typically won't care, but if you make a big public deal about screwing them over they'll catch on.
BTW, as long as you have satisfied any existing contract obligations regarding your current phone plan, you can keep your phone number if you change companies. In fact, even if you had another year on your plan, as long as you paid off the phone company according to their cancellation terms (usually a bad idea in the first year but it decreases dramatically in the second year of a two-year contract) you can move your number. The situation where the old company won't release your number is typically when you try to duck out of the contract; the old phone company will refuse to release the number as an incentive for you to pay them off, like you agreed to do.
It should not be a problem to move her over to your parents' plan. My brother has his own house and recently transferred his number from his own account to our parents' plan and just offered to pay our parents the difference after adding his line.