The phone number entered by Apple's mobile phone is invalid.

Mobile phone arrears, it is recommended to pay for recharge;

2.

Call forwarding, flight mode, call barring, etc. It's all set on the mobile phone. It is recommended to cancel and try again.

3.

Abnormal function or signal of mobile phone. It is recommended to change your mobile phone or location and try again.

4.

If roaming to other places can't be used, it is recommended to check whether the local network signal is normal and whether roaming permission is enabled for this number;

5.

Try to restart or change your mobile phone to see if you can answer the phone.