Section 11 of the quick start guide does mention possible interference with "Implanted medical devices" so I think your suspicion is correct. The guide suggests keeping the Doro Easy 508 more than 15cm from the medical device or not to carry the phone in a breast pocket or in a belt holder etc., that might contain metal. Perhaps your sister can wear her pendant round her waist or on her hip in order to reduce or even eliminate any interference problems .
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