1, the cell phone has no signal. If the dialed mobile phone can't receive the signal temporarily, there will be a prompt tone that can't be connected temporarily at this time.

2. Whether call forwarding is set. If the caller has set an empty number for call forwarding, the system can't find effective information when calling the other party.

The other party took out the SIM card without turning it off. If the owner forcibly takes out the SIM card without turning off the phone, and the system can't search the owner information when calling the other party, this prompt tone will appear.

4. The other party refuses to answer during the call. If the other party refuses to answer the phone during the call, the system will also appear.

5. The other party takes off the battery and is charging it.