How to Limit Calling to International Plan Coverage Limits

Hi -
We are a hostel (Venice Beach, California) and want to provide FREE international Voip calling to our backpackers / guests - as a benefit & service.

ISSUE:
We need to live within the base monthly cost to us of a voip provider's "all-you-can-eat" international calling plan's coverage limitations. (So that we know & control our cost for providing this benefit to guests.)

PROBLEMS:
(1) The provider's base monthly coverage cost excludes calls to international cell phones.
(a) In other words, there are additional minute / connection charges for these calls.
(b) NO VOIP PROVIDER will limit the ability to make calls just to what is covered by their respective plan(s).

(2) We need to either:
(a) EXCLUDE guests from making such calls altogether ;
(b) or CHARGE these calls to guest.

(3) If we charge guest: we need to take advance CASH "deposit" and then have call AUTOMATICALLY TERMINATE CALLS when guest reaches the deposit limit. (This exists in a few (expensive) analog call accounting packages.)
(a) Many backpackers do not have credit cards, so credit card solutions are not adequate; and unfortunately, would exclude many guests. It is also expensive added cost for credit card usage.

Thanks in advance for your help and guidance.
If not already obvious, we are a simple, budget travel alternative for (mostly) foreign & young travelers and would like to keep it that way. Additionally, we as a small operation have no technical expertise and no way to afford "corporate, professional" "solutions."

Mark
Venice Beach Hostel
info@planetvenice.com