Hi Badger,
Without knowing what model it is (you didn't say!) the only thing I could think of it you wiring a battery into it but then you'd need to know what you're doing.
A UPS may not be a bad option - if you wrongly wire the battery suggestion above then you may say bye bye to your phone, a UPS from http://www.ebuyer.com/UK/product/113906 is only £44 + carriage. May keep the phone going long enough for you.
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