The field comes from the SIM. However no modern device uses this field anymore.
Some carriers used to configure it, but that is pretty rare now.
Android considers this a protected field, so there is no way to program this field on the SIM unless you jail break the phone.
And a horrible fix I have some customers do; they put the SIM card into an iPhone, let it start once, and then put it into the Android device. Apple devices update the SIM field after they connect to the network.