Hi we have Storyphones and were caught out just like you... we even had £50 credit that hadn't been spent but when the company went bust and sold to the next owners (who are in fact the manufacturers Opsol UK of the Storyphones) we were told that we had lost all! The original company went bust about 3 years ago are you saying that Opsol have not gone bust?
We bought another set last year and have experienced nothing but problems which the sets; despite the company's efforts to fix them the company have not been able to put the problem right. They even gave us a replacement set for free but that didn't work either so we are stuck with two brand new sets that we cannot use :-(
You could try recording your own stories onto the phones if you have the little story phone recorder (often bought with the 6 way system). You can find out how to do this in your manual. Hope you have luck as when they are working they are terrific and much sturdier that the TTS version.