[Hilo] Why <AT+CHLD=4> returns “+CME ERROR: 257”?


Answer: <AT+CHLD=4> is to enable ETC (Explicit Call Transfer).
This service allows a user who has two calls to connect and release its connections to both other parties.
For example:
A call B;
AT+CHLD=2 to hold B
A call C;
AT+CHLD=2 to hold A
AT+CHLD=3 to activate A/B
AT+CHLD=4 to “Explicit Call Transfer”, then B and C is connected, then A will quit
The ECT function needs the support of the SIM card. Please check with your operator if possible to enable it.
The function is not so normal. As we know Orange and Vodafone support it but not supported by China Mobile.