Well, as with other markets (particularly web hosting), "unlimited" does not always mean "unlimited"....it often means "until we think you're using too much, and then we can take advantage of the vague portions of our Terms of Service and boot you at our own discretion". Each carrier may end up having their own "soft limits" they set for red flags to be bouncing up and then, if it continues, consider booting the customer. Unless we hear some particular experiences out there, it'll be just tossing darts at the dartboard.
However, to do some math on things... 40,000 divided by 30 days is about 1333 minutes a day. 1333 divided by 24 hours is about 55.55 minutes an hour. In reverse, 60 minutes x 24 hours a day is 1440 minutes a day. 30 days a month is 43,200 minutes. 44640 on 31 day months. If a person is using 40,000 minutes a month, they are connected almost constantly! Not sure how they recharge the phone.
5000 minutes a month is about 166 minutes a day in a 30 day month. That's about 2.75 hours a day. Which is in the ballpark of what many phones have for a battery charge while in use. Likewise, 5000 minutes at say .02 a minute is about $100 value a month. I think .02 is the cheapest even Verizon does on their prepaid daily charge accounts. Depending on what a person is paying, that may be reasonable in the end. Yet, it may not.
100 minutes a day averages to about 3000 minutes a month. What would a person have to pay to get that?
To be blunt, I *hate* these unlimited designations that are anything but. If it's not truly unlimited, it should not be labeled as such.