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Your handset may display different "network operator" names depending on where you are located either domestically or internationally. Your handset depending on the handset itself and what is "broadcast" from the nearest tower may not always show T-Mobile. It may show the name of a roaming partner if you are in an area where T-Mobile does not have native service.
In California and Nevada depending on your handset and your SIM your handset may show Cingular since the network in those locations was formerly shared with Cingular Wireless. The network now belongs to T-Mobile but Cingular subscribers still use the network and it's likely that when Cingular transitions their subscribers to the former AT&T Wireless network (the so called "blue" network") the network will continue to show as belonging to cingular. It's likely that when all the cingular subscribers are transitioned to the new network that T-Mobile will change the network mobile code and at that time all handsets will show T-Mobile and will not show Cingular any longer.
Because T-Mobile was built from several networks, as were nearly all the national carriers, it is not uncommon for a phone to display 'TMO among other network operator names in some regions instead of T-Mobile. This does not affect your phone's operation or change the way you will be billed.
"6.15 MOBILE NETWORK CODE.
(a) T-Mobile will broadcast MNC 170 only on the California/Nevada network beginning at Closing and ending on the earlier of Cingular using 60 billion minutes of airtime (measured on the same basis as contemplated to be measured in the Wholesale Agreement) on the California/Nevada network or two (2) years from the Closing (the "INITIAL PERIOD"). T-Mobile will have the option to broadcast MNC 170 for up to one more year following this Initial Period (the "FOLLOW-ON PERIOD"). T-Mobile shall notify Cingular no later than sixty (60) days prior to the end of the Initial Period of T-Mobile's intent regarding the option.
(b) No later than six (6) months following the Closing, Cingular will cease broadcasting the MNC 170 code in the Seattle market (and any other market where it broadcasts MNC 170) during the period when T-Mobile will broadcast MNC 170 in California/Nevada. During the period following Closing through the time when Cingular ceases broadcasting MNC 170 in the Seattle (and any other) market, in such markets Cingular will LAC-restrict for international roaming traffic a minimum of 25 countries (the "RESTRICTED LIST") specified by T-Mobile as set forth on Section 6.15(b) to the Cingular Disclosure Schedule. Cingular will use commercially reasonable efforts to add more countries to the Restricted List in the order specified in the Restricted List.
(c) During the Initial Period and any Follow-on Period, Cingular will take all actions reasonably requested by T-Mobile in order to ensure that T-Mobile has the right to the benefits of owning MNC 170, including the right to offer third-party roaming and obtain all revenues associated with broadcasting MNC 170 in California/Nevada, including, without limitation, communicating with any roaming data clearinghouse, any financial clearinghouse, and any third party roaming service provider.
(d) Also during the Initial Period and any Follow-on Period, (i) T-Mobile will continue to broadcast Cingular's alpha code with the MNC 170 as currently in operation in California/Nevada and (ii) T-Mobile and Cingular will use commercially reasonable efforts to optimize T-Mobile's customers experience regarding the alpha display.
(e) T-Mobile will not sell SIM cards with the MNC 170 as the home network."
Handset "SIM unlocking"
Your handset is most probably "SIM locked" if you obtained your handset from T-Mobile meaning that the handset is locked to only allow use of T-Mobile SIMs. Handsets are locked because when you got your handset you perhaps got your handset for free or at a discount off the entire price that the handset cost. T-Mobile and other carriers commonly lock their handsets so you cannot use your handset with a competing GSM service. T-Mobile's policy is that they will attempt to supply you with the unlocking code for your handset provided you have been a subscriber over 90 days with no lapses in payment of your account.
If you wish to get the unlock code for your handset call customer care (611 from your phone or 1-800-937-8997) and explain to the customer care representative that you wish to obtain the SIM unlock code for your handset. Make sure that the representative understands that you wish to get the SIM unlock code and not the PUK code for a locked out SIM. The representative will ask you for some information. They will ask you for the phone's "IMEI" or serial number. You can get this number either on a sticker under the battery on your phone or you can key *#06# and the code will be revealed to you. The representative will put in a request to get the unlock code. Usually you will be contacted within 48 hours with the code and the instructions on how to apply the code. You should be aware though that sometimes T-Mobile is unable to immediately get the unlock code from the manufacturer.
If T-Mobile is unable to get the unlock code for you there are sites on the internet where you can obtain software to calculate the code. Also there are people who have businesses that specialize in unlocking SIM locked handsets.