I did it and it seemed to work.
First call MCI business services to cancel your current service (it takes 7-10 biz days - so don't worry about being without a phone).
Explain that you want to be transferred to "residential" (because you're not using the phone for a business anymore - or because it's a better deal or whatever).
After some time on hold, you will get residential. Pick one of their deals (check their website - or they have a lot of options not on their website -- eg. different variations on "anytime".).
But first tell them you want the $75 free usage promotion. As previously described, this is a credit up to $25 for the next 3 months on your phone bill.
I don't think its cumulative, so if you don't spend $25, too bad.
Seemed to work for me, but the phone service won't get switched over for a week.
Also, I don't think the $25 is deducted automatically each month (that remains to be seen). I think they send you vouchures in their "welcome pack."
I think this $75 offer appeared in an American Airlines brochure. One of the residential services clerks said something about frequent flier miles as an alternative, but I wasn't interested so ??.