I'd log into your account with your ISP. They should have a section detailing how much you've used.
I just did this a couple days ago and for the first time and found I've only used about 18 gigs out of 300 with four days left within the cycle. Right now I surf and game, but am thinking of dropping my cable TV, hooking up my antenna on the roof for local HD, and signing up for a couple streaming services. Its good to know I have about 220 gigs to play with.
Also, what router do you have? If your router supports it, you could install third party firmware on it such as Tomato, which can monitor bandwidth consumed by each device on LAN, both real-time and historical.
You want a program called NetLimiter.
You can track your bandwidth "on the fly" You can throttle connections, and in the gaming community, some use it for intentional "lagging" in games.
They have a trial version if your not interested in purchasing now, but I highly recommend the full version. Very stable, and being currently developed, Windows platforms only!
That's great for a PC, but how would he install it on Ooma device which plugs directly into the router?
The only way to measure its data usage is at the router level. Unless you have a PC that you're using as a router...
It would be a great project. A cheap PC is a dime a dozen, you could probably build a P.C. router for under 50 bucks.
I agree that it would be a neat project, but I highly doubt you can do it for under 50 bucks. You'd want a motherboard with two NICs to begin with.
Then there is the power consumption issue. A typical PC will chew up 100W, 24x7.
VOIP services like ooma use basically nothing. Watching a single 3 minute youtube video likely uses more data than the average person would use in a month via ooma. It's not enough worth worrying about.