Depending on your choice of VoIP providers, you could probably do this already, as some have a callthrough/bridge arrangement like this
eg. I have voipfone and voipbuster accounts. My incoming voipfone number is enum-enabled so could be reached via Sipbroker pstn access. I could forward that voipfone account to either voipfone or voipbuster (or any other) callthrough that recognises the (voipfone number) caller ID I've pre-registered, get a dialling tone, and dial out.
There's a double charge though, or treble counting the original access call, so it's probably as easy to just use an ordinary calling card costing 2c a minute. Or use a callback system, which is where we came in.
Using callback means the chance to define provider(s) for both legs, chosen for good tariffs, whereas callthrough means you are stuck on the tariff of the outgoing access call.
How many callboxes publish their number and accept incoming calls? Using callback here would be much cheaper than stuffing coins in or dialling a freephone access number that surcharges your calling card account.
At the moment the Voxalot mobile callback is used only for numbers that are preset in the phonebook. Maybe finding a way to enable it for undefined numbers would be as flexible as above, without the cost of extra DIDs or as much need for programming of caller ID recognition and dialled-in passwords, which are fairly akin to calling card methods. But I'm not objecting if you decide to do it
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