Quoting krishnakant Mane researchbase@gmail.com:
your next point is some thing tricky. let me give you an example. if I give you a mobile handset and say that only huch will work on this and no airtell will you buy such a phone?
You do get such phones in the marked(called locked phones). These handsets are generally given for very nominal price. Such an arrangement works in favour of both customers and operators.