Network sharing agreement
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From 2001 to 2004 T-Mobile (at that time VoiceStream Wireless) entered into a network sharing agreement with Cingular Wireless that would give Cingular the ability to have customers in the metropolitan New York City area to use Cingular. In exchange Cingular would allow VoiceStream to use their networks in California and Nevada to use their network. This network sharing arrangement came to an end when Cingular bought the AT&T Wireless network. The network used by Cingular in California was sold to T-Mobile USA with the stipulation that Cingular could continue to use their legacy network for a number of years until Cingular transitions all their subscribers to the former AT&T Wireless network.
Source(s): Network sharing agreement