The term box breaker pertains to the mobile phone industry; where there are companies or individuals that exploit the networks' use of discounts on handsets to create their own profitable business.
The assumption is that a customer will buy the handset, and then activate with the SIM-card in the pack, which will eventually lead to the operator making money via top-ups on the phone.
Additionally, online marketplaces (like eBay) enable box breakers to operate internationally and exploit fluctuations in currencies and price differences between countries.
While chains like O2 and Carphone Warehouse have clamped-down on sales to box-breakers, there are many other outlets, including catalogue stores and supermarkets that have not addressed this phenomenon.
[2] Mobile phone handsets that are in high demand - especially models that are region-exclusive - are prime targets of box breakers.