Some older model iPhones were not cross-carrier compatible. For example, the AT&T iPhone X was not compatible with the Verizon network. So, even if you have an unlocked iPhone, ensure that the carrier you want to activate it on is compatible with the model number you have. The model number is listed on the back of your iPhone and looks something like a letter and a number combination (for example, 'A1660' indicates some iPhone 7 models).