This stems from a comment by faye. Seems that the phone usually detects the values from the SIM card as my Android phone auto populates the MCC and MNC fields but for some non-mainstream phones these values may need to be manually entered
A Mobile Network Code (MNC) is used in combination with a Mobile Country Code (MCC) (also known as a “MCC / MNC tuple”) to uniquely identify a mobile phone operator/carrier using the GSM, CDMA, iDEN, TETRA and UMTS public land mobile networks and some satellite mobile networks. The ITU-T Recommendation E.212 defines mobile country codes.
The following tables contain the complete list of mobile phone operators. Country information is provided for completeness, though this information is best obtained from theList of mobile country codes page, where ISO 3166-1 country codes are also cross referenced.
The MNC and MCC values within the table are decimal numbers.