shill sometimes produces some confusing log messages related to MCCMNC lookup:

>> WARNING:mobile_operator_info_impl.cc(481)] Unknown MCCMNC value [31148].

This one occurs when shill hasn't determined the MCCMNC for the MNO through a 3GPP registration update, so it falls back to extract MCCMNC from IMSI. As MNC could be 2 or 3 digits, shill tries the first 4 and 5 digits of IMSI.  One of the them will result in the unknown MCCMNC warning above.

>> WARNING:mobile_operator_info_impl.cc(481)] Unknown MCCMNC value [].

This one occurs when MCCMNC is cleared due to a registration loss, so shill is looking up an empty MCCMNC value.