I've been doing this for a while, and to the best of my knowledge there are no tools in the wild to clone a phone. These kind of tools, exist at the government level where agents of governments can create a clone phone, with the info the telco has about your phone. But end users don't have access to these tools. Such phones are very quickly noticed by most telcos, who take actions against them.
As I said there are areas of the phone that are not written to, when the phone is restored, additional areas (modem firmware) are written to on a phone update. But even with a phone update, the entire memory of the phone is not rewritten. The IMEI (International Mobile Equipment Identity) and the ICCID (Integrated Circuit Card ID) are not written to. There is also an UDID number, Service Provider Name (SPN), and Local Area Identity (LAI), lots and lots of memory is not changed. Then there are MAC addresses on your iPhone, one for Bluetooth, one for Wi-Fi..... These also don't change on a restore or update.
I suppose it is worth while to remember that your service provider has the ability to reach out and touch your phone and disable it at will. To turn it into a brick....
As to unlimited data plans, that is simply, hype for the most part. And every service provider that I am aware of treats mobile hotspot data different than data used within the phone. Phone companies make money from overages charges, or the threat of them and are very creative when it comes to finding ways to put us over. My service provider, as an example requires me to choose between unlimited data on my phone, or the ability to turn my phone into a hotspot.
****If your iPhone is not your iPhone, but rather your company's iPhone you might also want to stop and think about how they would feel about jail breaking it. Many companies take security very seriously when it comes to their LANs. Many see a jail broken device as a security breach and grounds for dismissal
some software refuses to run on jail broken devices, iBooks is one as well as some Cisco apps. *****