You are losing signal and cannot connect to the network. Signal strength may be affected by weather or other conditions. You might find the X disappears when you change location, for example. So, the problem may be in the environment, in the network, or in the SIM card. If it is a bad SIM card, such has to be replaced. If it is a network problem, you would have to contact your service provider. And you can't do much about the weather other than try the phone is a different location. Generally, a red X means a function is not available and you have to go into setup to enable the function. But in this case, you are having network connection problems where the phone cannot detect the signal.
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You are losing signal and cannot connect to the network. Signal strength may be affected by weather or other conditions. You might find the X disappears when you change location, for example. So, the problem may be in the environment, in the network, or in the SIM card. If it is a bad SIM card, such has to be replaced. If it is a network problem, you would have to contact your service provider. And you can't do much about the weather other than try the phone is a different location. Generally, a red X means a function is not available and you have to go into setup to enable the function. But in this case, you are having network connection problems where the phone cannot detect the signal.