callback service comparable to callbackworld?

[inquisitor]

Originally Posted by petkow

There is also an SMS way of doing this but I haven't explored that option yet.

Really? I used SMS-triggered callback until sommer 2007 when Betamax ceased that feature at least for justvoip and poivy. But I remember a Polish guy on the voipbuster forum, who had built a public SMS-callback-gateway afterwards. Maybe his service is still online, but I wouldn't share my credentials with an anonymous individual.
However IP-based triggering is probably cheaper than SMS, if you have a SIM with reasonable data roaming tariffs. With moipe.com, a data roaming discounter who will launch soon, a callback request would cost € 0.09 (for 10KB), while SMS are always way more expensive while roaming.