I think you are doing fine. I was very relieved to read the tone of your post and almost all of the replies -- all respectful, polite, and solution-oriented, not the sort of "Oh, goody, an excuse to really give it to someone -- I get to be screamingly mean, because they were rude! Yay!" tone we sometimes hear.
And I agree with the advice. Make a policy of NO calls, to or from anyone, not just MIL, during dinner. Let her know in the same polite way you've been doing. Turn off the ringer during dinner if necessary.
The only reason I am posting too is to add this: make sure you do return her calls as promptly as possible, and without letting her hear your exasperation. Don't get sucked into any conversations about why you didn't answer, either to apologize, defend yourself, or to lecture her, just as quickly as possible get to, "Well, here we are now -- what's up?"
Some people who call insistently are controlling, but others are simply insecure or lonely. If she feels like she gets enough of her family's attention, she will be more likely to back off. I am not saying that you and your husband have been neglecting her or pushing her off (it doesn't sound a bit like you are), and I definitely recognize that some people are bottomless pits for attention. I just mean that I have learned that as a general principle, what works best with people who seem so hungry for attention is not to pull away, but to make sure to give them plenty of it, on terms that suit me, and then they are much, much less likely to demand it in inappropriate ways and times. I know that doesn't work in all situations, but when possible, try it.