There is a framework available (CoreTelephony) that provides that information, however I don't think it has been implemented in B4i. There is an example here -> How to read missed calls or check phone state? on how to use CoreTelephony with inline Java, so that might point you in the right direction.
Dim netinfo As NativeObject
netinfo = netinfo.Initialize("CTTelephonyNetworkInfo").RunMethod("new", null)
Dim carrier As nativeObject = netinfo.RunMethod("subscriberCellularProvider", Null)
Log(carrier.RunMethod("carrierName", Null).AsString)
Dim netinfo As NativeObject
netinfo = netinfo.Initialize("CTTelephonyNetworkInfo").RunMethod("new", null)
Dim carrier As nativeObject = netinfo.RunMethod("subscriberCellularProvider", Null)
Log(carrier.RunMethod("carrierName", Null).AsString)