Elderly people and especially people with dementia often experience social isolation and need assistance while performing activities of daily living. We investigate a novel approach to cope with this problem by integrating voice assistants and social assistance robots. Due to the special communication needs of people with mild cognitive impairment, the design of interfaces of such systems is to be based on the particular requirements of the target user group. This paper investigates, how a voice user interface should be designed for elderly users with mild cognitive impairment - such as an early stage of dementia - to provide personalised support throughout activities of daily living. A context and user analysis delivered a set of 11 guidelines for voice user interfaces for people with dementia. For a pilot study we selected those strategies often applied by caregivers in their communication with people with dementia and evaluated the voice user interface among elderly participants and healthcare workers who reported a high feasibility, usefulness and acceptance of the designed system.