Yes, an update to the Preferred Roaming List, which specifies which networks the phone is allowed to use. Always includes the carrier's own networks, and includes roaming networks selected by the carriers - usually the networks run by companies with which your carrier has the most favorable roaming agreements.
This is a CDMA thing. So you'll have PRLs on Verizon, Sprint, Alltel/Western Wireless and US Cellular phones, as well as phones from a few smaller CDMA carriers, GSM uses a different system to identify where the phone is allowed to be used.
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