Quite often, Android users encounter the problem of blocking incoming calls. The phone does not display information about the incoming call, but your interlocutor receives automatic message about the subscriber's unavailability on the network. Of course, Android does not accept incoming calls and deprives the phone of its main function - the ability to connect two interlocutors. This problem may occur due to a malfunction software smartphone, breakdown GPS module or if there is a problem with the settings. Let's try to solve the problem when Android does not accept incoming calls.
Airplane mode.
When setting up your phone, you may accidentally turn on airplane mode. Selecting this profile is characterized by the fact that all active networks will be unavailable, but the phone will work. You will not be able to call, send SMS or use the Internet; incoming calls will also be blocked. Offline mode Widely used during flights, as well as to preserve battery charge. To disable this mode you need to do a few things: simple operations: hold down the power button of the smartphone, turn off Flightmode.
The SIM card is faulty and the Android is not accepting incoming calls.
There are times when the SIM card barely touches the phone contacts. This may be caused by a broken slot, unusable card, or loose fasteners. If the card is removed, the network on the phone will be lost and you will not be able to use communication services. A message indicating that there is no SIM card will be displayed on the screen. Your actions in this situation are extremely simple: take out the card, wipe the chip, insert the card back. Did it work? Great. If the network is still not identified, then the problem is most likely in the slot. To fully verify that the equipment is not working, you should insert another card. If there is no network, then we can safely take the phone in for repair.
There is no signal and the Android may not accept incoming calls.
The coverage area of each operator is different. Don't be surprised if your phone doesn't recognize the network in the forest - it's justified weak signal. Also, most phones begin to lose the network in large ravines, remote places, and basements. To restore the network, you need to find a coverage area and the Android will again accept incoming calls.
There are also a number of internal problems that cause network loss. There is no way to deal with such breakdowns at home, because you need special equipment and spare parts. To carry out repair work, it is better to contact a specialized company that will repair your phone.
An attacker can carry out an attack to disable the phone, and the phone user will not even know. At the same time mobile device will be included. The mobile device will not be able to receive incoming calls and SMS messages. On the phone screen, the communication strength indicator will show the maximum value. To implement such an attack, attackers do not turn to mobile operator. An attacker can carry out such an attack with little effort. The only thing that gives away the attack is fast discharge phone battery. The phone will drain quickly. Some phone models turn on the phone's backlight when exposed to an attack, and it does not turn off while the attack is in progress.
Such attacks create threats that could lead to disruption of important negotiations or critical situations when a work phone is needed. Sometimes such attacks are only part of big plan relative to the user.
How do attackers carry out such attacks?
The article about SMS spam describes the operating principle of SMS services. But few people know that such services can implement undocumented capabilities. Eat special type SMS messages - invisible SMS. Such SMS are not displayed on the phone screen and are not notified by the phone that it has been received. Such SMS are usually used by operators to check the existence of a mobile phone, but so that the user does not know about the request. Attackers can use this to carry out a DoS attack on a mobile phone.
The implementation of a DoS attack through invisible SMS is implemented because SMS messages implement the signaling layer in the GSM network. Such messages clog the signal channel. You can create generators for sending invisible messages using HTTP or SMPP protocols. The basis of invisible SMS includes simple circuit. Table 1 shows the SMS message format. Using these parts, an SMS is generated and sent to the SMS center.
Table 1.
| Field | Size in octets | Type | Description |
|---|---|---|---|
| command_length | 4 | integer | Entire message length |
| command_id | 4 | integer | ID of the entire message |
| command_status | 4 | integer | Status |
| command_number | 4 | integer | Unique sequential number |
| service_type | 4 | string | Service type or Null |
| source_addr_ton | max 6 | integer | Sender number type |
| source_addr_npi | 1 | integer | Sender indicator |
| source_addr | 1 | string | Sender's address |
| dest_addr_ton | max 21 | integer | Recipient number type |
| dest_addr_npi | 1 | integer | Sender ID |
| dest_addr | 1 | string | Recipient's address |
| esm_class | max 21 | integer | Message Type |
| protocol_id | 1 | integer | Protocol ID |
| }
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