That is true for the iPhone 4, but not the 4S.
For the 4S, go into Settings --> General --> About and look at the model number. Then compare it to this (lovingly lifted from
Everymac.com)
MC918LL/A is the order number for the 16 GB configuration in black locked to AT&T with a two-year contract. With 32 GB or 64 GB of storage, in black, locked to AT&T, the order numbers are MC919LL/A and MD269LL/A. With 16, 32, or 64 GB of storage, in white, locked to AT&T, the order numbers are MC920LL/A, MC921LL/A and MD271LL/A.
Locked to Verizon, in black, the order numbers respectively are MD276LL/A, MD278LL/A, and MD280LL/A. Locked to Verizon, in white, order numbers are MD277LL/A, MD279LL/A, and MD281LL/A.
Locked to Sprint, in black, the order numbers are MD377LL/A, MD379LL/A, and MD381LL/A. Locked to Sprint, in white, order numbers are MD378LL/A, MD380LL/A, and MD382LL/A.
Finally, the order numbers for unlocked models are MD234LL/A, MD241LL/A, and MD257LL/A in black and MD237LL/A, MD244LL/A, and MD260LL/A in white for 16 GB, 32 GB and 64 GB, respectively. Although the unlocked model is a "world phone," Apple unfortunately has disabled its CDMA capability in software.
For example, see the screenshot from my phone. It's a white, 32GB AT&T 4S with a model number of MC921LL, just as it shows up in that list