This is not a problem actually. The operator you're using has a roaming pact with another operator. So if you're under bad network coverage or no network coverage your phone will search for other available operators And register you with the one that has a roaming agreement with your service provider and you'll get that warning. If none is found Then you'll get no network available message. to turn off this feature go to settings> wireless & networks> more>mobile networks> network operators and set the network search to manual then select your home network.
