A unique instance id of the user agent. This is the instance.id defined in RFC 5626. According to RFC 5626, each UA MUST have an Instance Identifier Uniform Resource Name (URN) [RFC2141] that uniquely identifies the device. Usage of a URN provides a persistent and unique name for the UA instance. It also provides an easy way to guarantee uniqueness within the AOR. This URN MUST be persistent across power cycles of the device. The instance ID MUST NOT change as the device moves from one network to another. A device like a "soft phone", when first installed, can generate a UUID [RFC4122] and then save this in persistent storage for all future use.

public string UserAgentInstanceId { get; set; }