Apologies if this is a FAQ type of question... but..

Is it possible to trigger a callback onto mobile phone A by dialling a number which doesn't answer on mobile phone B - avoiding a double call back?

It doesn't matter what phone you call the did (callback number) from to trigger the callback. The number you dial is what triggers the custom callback to the number you have set up and chosen. The inward number is specifically linked to the number to be dialed. You can even trigger the callback over the internet.

I do this regularly. In places like the Carribean, this is actually the best way to go because Cable & Wireless tries to stop callback services by using a program looking for an incompleted call followed by an immediate callback.

This is the "two phone method" I use with some international cards, like Riiing. You use your roaming postpaid phone to trigger a callback to your Riiing SIM and bypass the higher rate for outbound calls.

slight reservation - there are one or two countries where the trigger call from a second phone seems to be charged [Slovakia, Lithuania?], not nice on per minute billing. In Slovakia (last year), the double callback from Riiing was not charged though