Ok, so I just called. First, employer discounts do NOT apply to this, so the price is $10 no matter what.
Second, the rep confirmed that calls and texts are included if they are FROM an approved country to the USA, or from that country within that country. If you are in 1 country and call another, you pay as you would from the USA (LD charges).
Therefore, the only risk for "overage" is if you exceed your data plan, or you call/text from USA to Foreign, or Foreign 1 to Foreign 2.
You need to enable it on your account, but it can be done at any time - you aren't billed unless you use it. No need to add and remove it as with the old plans.
In my case above, this will fill the gap nicely while I'm in LHR and until I get to the Meteor store to get the new SIM. Then I'll switch over for the rest of the trip.. so $32 + $10 isnt that bad for 2 weeks of virtually unlimited usage.