Modern smartphones have a central repository of contacts managed by the operating system. That is used for normal telephone functionality, but it's also available to apps via an API on each platform.
Zebble gives you a unified API to query the device contacts which works across all platforms.
Get all device contacts:
Searching for specific contacts:
// It may be an empty list if no result is found, or it may return multiple contacts.
The result of both methods shown above is an IEnumerable<Device.Contact> objects.
Each contact object has many string properties as well as lists of objects: