The question is frequently posed, "How do they do this", and (especially about the VoIP services with 'free' destinations) "What's the catch?"
All of these services require pre-payment; i.e. you purchase a certain amount of credit using PayPal or your credit card, and can then make calls until your credit is used up. For most users, credit is also necessary for free calls: unless you have credit, your free calls are limited to 1 minute per call.
If you only make calls to free destinations, your credit will not be used up; but unless you purchase more credit within 120 days (i.e. 4 months), your remaining credit will expire. Since the system will not let you purchase more credit unless your balance is below EUR 5, your remaining credit definitely will expire unless you use it.
Thus, the company realizes revenue even from users who only make free calls. Since the minimum top-up payment is EUR 10 (in some cases initial payment is EUR 5), if all you call is free destinations, you are effectively paying EU 2.50 per month for a very economical flatrate.
A lot of customer unhappiness is generated when any one of the VoIP services, where a user has a relatively high balance, stops offering free calls to that user's favorite destination, while one of the other VoIP services still offers them; since there is no way of transferring credit from one Betamax service to another, it means either using up the credit on the first service (and thus not making free calls), or allowing that credit to expire while purchasing credit for the service which still offers free calls.
No doubt that is one of Betamax' ways of maximizing its revenue, and while it may rankle, it is generally still the best deal to simply use up your credit on no longer free (but in all likelihood still cheap) calls with the first service before signing up for another service (unless you need multiple accounts for some purpose).
Another irritation is the fact that not only credit expires 120 days after the last purchase, but so does the free VoIP-IN number associated with the account. Thus, if you simply forget to top up, you may loose your number and probably will not get the same number back.
I believe that many of these irritations could be avoided if Betamax simply offered a flat rate package, charging, say, EUR 3/month, for VoIP-In number and free calls to ALL of their free destinations, with the option of setting up recurring payments, thus avoiding any mishaps. But for some reason they don't seem to want to do this.