Dear, the answer is as follows: 1. Your environment is not in the signal coverage area, such as the top of a mountain or a remote suburb. Suggest trying another area. This will happen when the mobile phone system crashes. Need to restart the phone. 3. If conference mode or flight mode is set, you can't access the mobile network, just turn off the flight mode. 4. The phone card is in arrears or the phone card is out of order, resulting in abnormal calls. If you owe money, you can pay the phone bill. If the phone card is out of order, you need to find a business hall to test it. 5. If the calling card on the mobile phone does not match the mobile phone, it will also show that the mobile network cannot be accessed. It is recommended to replace other mobile phone cards as much as possible. 6. The mobile phone card does not support or enable the VOLTE function. Just enable the VOLTE function. 7. If roaming to other places can't be used, it is recommended to check whether the local network signal is normal and whether the number has been roaming.