This is a VERY old subscription - fully paid up in 1996.
This may be true, and at the time might have costed a fair bit, but it's like buying a car say in 1996, it is now 2005, and what you paid for back then, has devalued considerably.
Unless you really need that old ISP, there are plenty around that will for free of charge provide a geographic number. Although some might get caught in that trap, of not being able to be called once enough telcos ban the number, if you don't mind switching every now and then, and of course not paying any subscription fees, then it might be worth doing that, instead of sticking to the line that you paid for something which was back then, in 1996 worth something, but with evolving technology and prices, not worth anything.
I'm not saying you were ripped off back then, because the norm was to pay a fixed monthly fee, and then call charges. Back then very few telcos offered free calls, and 0845 was cheaper than phoning a geographic number outside the local area.
I think the best idea is review what you want from your ISP, and just cut your losses. Do you want to be stuck with a number you can't call, or would you rather look around for something else?