If you're experiencing issues with your Android phone that relate to the mobile network, the most likely cause is an invalid MMI code. This code is used by your carrier in order to identify the type of device you are using and the service you are subscribing to. To troubleshoot and fix the issue, try re-pairing your device with your carrier or using a different SIM card. If that doesn't work, then it may be due to poor signal strength, dropped calls, or blocked calls.