In other words, before you bought the phone, somebody tampered or performed additional work on the equipment, after it left the manufacturer. These phones are normally co-branded with the service provider's logo. Because the service provider is guaranteed an income from the calls made on this locked phone, they can sell the phones at a much cheaper price. Should you buy such a phone and want to use it with the MTC network, the cellphone will have to be unlocked by the network that locked it in the first place - at a price of course. A SIM locked phone is normally in combination with a pre-paid SIM card.
Many service providers sell cellphones with a pre-paid package at reduced costs.
However, once you have inserted the SIM card into the phone, it will only accept that particular service provider's SIM card - unless it is unlocked, which will of course also cost you money. Apart from the financial expense, you will also have the trouble of sending the phone back to the country where you originally bought it. Big mobile operators order huge amounts of handsets from a particular manufacturer, which allows them to brand the handset. In conjunction with that, the manufacturer may also either network lock or SIM lock the phone for the operator. So, next time you go shopping for a new cellphone, make sure to opt for the better buy - you will save yourself a lot of trouble and your phone has a legitimate warranty period.