Most mobile devices that are used for placing mobile telephone calls can be identified by the IMEI number (International Mobile Equipment Identity) which is normally unique per mobile device. However certain vendors use the same IMEI number in more than one device, this being a potential issue for tracking.
Using the IMEI or IMSI number or telephone number of the device for reliable analytics is possible when the analytics provider is hooked up with the appropriate telecom network providers, while true in the good old days it is not any longer. Today mobile devices hook up to the internet using multiple ways.
Ever since the mobile devices were equipped with WiFi capability, tracking uniqueness based on the IMEI or phone number suddenly is less than accurate.
If a device accepts cookies then there is yet another uniqueness factor to take into account to use, but this means there is not always a connection between the uniqueness identifiers. And browsers like Safari in a mobile treats 3rd party cookies without mercy.
To add to the mix, recall the WiFi hotspot functionality of the iPhone 4 enabling it to act as a gateway for other mobiles in cases where other mobile users choose to use a friend's device for internet access.
Plus any laptop or other device hooking up thru the same shared device means that uniqueness counting suddenly is more than a bit tricky.
Then when Smartphones hook up in a café, using the free WiFi these typically offer, while the Smartphone owner enters latte warrior mode for a while.
Suddenly the perceived uniqueness isn't there anymore as a single device could be identified by at least 3 different value identifiers, beyond that a potential 4th identifier being unique app (application) identifiers and to stir things up even more a 5th could be the unique device ID available for app's in Smartphones.
As is with browsers the abuse and misuse of Flash cookies, used as a bridge to pass uniqueness values, could have been very much having a field day on most Smartphones had Apple not avoided Flash and kept their user base off the dodgy bridge route.
In order to track user interaction correctly one must decide on which uniqueness value to use depending on what is being tracked. Is in an app or mobile site? Typically app's using the unique device id will get fare a better count on uniqueness, if the devices allow cookies then that will work as well keeping in mind cookie churn. User id if the user is logged in is another option.
For a mobile website the answer isn't as easy since there are many factors as mentioned above that influence the uniqueness used to count unique devices. Is the user connecting via telecom provider or WiFi? Are any unique values present, i.e. cookie, IMEI number, phone number, anonymous unique id etc?
Selecting the uniqueness key that gives the highest number is essentially shooting yourself in the foot, instead for the sake of accuracy evaluate which uniqueness key is essential for the website/app being tracked. Make sure to over time evaluate the constantly shifting behavior of the users as well as the communication methods that are available. The choice of uniqueness key will have a notable impact on the usefulness of the web analytical data any analysis is built on.
Out of privacy concerns the move to anonymous unique device identifiers in already happening. Just make sure to understand which unique key is used when the term unique mobile is floated, it is very elusive.