I have seen example similar to this with MS in idle mode. The MS re-scanned and camped on cells of V/fone and O2 at different times while MS in idle mode when the home network was T-Mobile. The answer was that the T-Mobile coverage RxLv was very poor (dropped below -114dbm) in the area of testing (rural) and performed re-scan. The MS wouldn’t make call on V/fone or O2 as there no T-Mobile roaming agreement with these operators.