Cloning used to be a lot easier. And in some countries, such as India, it's still a widespread problem.

In the old days, you needed only to get a couple of unique identifying numbers from the target phone and then enter them on a secret menu on the clone phone.

These days, it's very difficult. If you want to get the requisite secret identifying numbers, your best bet is to hack a cellular carrier's database or use expensive, specialized equipment to snatch the numbers from the airwaves (a technique that also requires physical access to the SIM card).

For the petty crooks who used to clone phones in order to sell phones that could make free calls (billed to the victim), cloning is an industry in decline.

Besides, the "benefits" of cloning are largely available through other means, such as free VoIP calls, and some of the techniques I detail below.